feat(translation): machine translation pipeline + admin bulk enqueue UI

- LibreTranslate client (chunks on blank lines, ≤4500 chars, 3-goroutine semaphore)
- Runner translation task loop (OTel, heartbeat, MinIO storage)
- PocketBase translation_jobs collection support (create/claim/finish/list)
- Per-chapter language switcher on chapter reader (EN/RU/ID/PT/FR, polls until done)
- Admin /admin/translation page: bulk enqueue form + live-polling jobs table
- New backend routes: POST /api/translation/{slug}/{n}, GET /api/translation/status,
  GET /api/translation/{slug}/{n}, GET /api/admin/translation/jobs,
  POST /api/admin/translation/bulk
- ListTranslationTasks added to taskqueue.Reader interface + store impl
- All builds and tests pass; svelte-check: 0 errors

backend/internal/backend/handlers.go

@@ -32,6 +32,7 @@ package backend
 //     directly (no runner task, no store writes). Used for unscraped books.
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -49,6 +50,7 @@ import (
 	"github.com/libnovel/backend/internal/novelfire/htmlutil"
 	"github.com/libnovel/backend/internal/pockettts"
 	"github.com/libnovel/backend/internal/scraper"
+	"github.com/yuin/goldmark"
 )
 
 const (
@@ -701,9 +703,252 @@ func (s *Server) handleAudioProxy(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, presignURL, http.StatusFound)
 }
 
-// ── Voices ─────────────────────────────────────────────────────────────────────
+// ── Translation ────────────────────────────────────────────────────────────────
 
-// handleVoices handles GET /api/voices.
+// supportedTranslationLangs is the set of target locales the backend accepts.
+// Source is always "en".
+var supportedTranslationLangs = map[string]bool{
+	"ru": true, "id": true, "pt": true, "fr": true,
+}
+
+// handleTranslationGenerate handles POST /api/translation/{slug}/{n}.
+// Query params: lang (required, one of ru|id|pt|fr)
+//
+// Returns 200 immediately if translation already exists in MinIO.
+// Returns 202 with task_id if a new task was created.
+// Returns 503 if TranslationStore is nil (feature disabled).
+func (s *Server) handleTranslationGenerate(w http.ResponseWriter, r *http.Request) {
+	if s.deps.TranslationStore == nil {
+		jsonError(w, http.StatusServiceUnavailable, "machine translation not configured")
+		return
+	}
+
+	slug := r.PathValue("slug")
+	n, err := strconv.Atoi(r.PathValue("n"))
+	if err != nil || n < 1 {
+		jsonError(w, http.StatusBadRequest, "invalid chapter")
+		return
+	}
+
+	lang := r.URL.Query().Get("lang")
+	if !supportedTranslationLangs[lang] {
+		jsonError(w, http.StatusBadRequest, "unsupported lang; use ru, id, pt, or fr")
+		return
+	}
+
+	cacheKey := fmt.Sprintf("%s/%d/%s", slug, n, lang)
+
+	// Fast path: translation already in MinIO
+	key := s.deps.TranslationStore.TranslationObjectKey(lang, slug, n)
+	if s.deps.TranslationStore.TranslationExists(r.Context(), key) {
+		writeJSON(w, 0, map[string]string{"status": "done", "lang": lang})
+		return
+	}
+
+	// Check if a task is already pending/running
+	task, found, _ := s.deps.TaskReader.GetTranslationTask(r.Context(), cacheKey)
+	if found && (task.Status == domain.TaskStatusPending || task.Status == domain.TaskStatusRunning) {
+		writeJSON(w, http.StatusAccepted, map[string]string{
+			"task_id": task.ID,
+			"status":  string(task.Status),
+			"lang":    lang,
+		})
+		return
+	}
+
+	// Create a new translation task
+	taskID, err := s.deps.Producer.CreateTranslationTask(r.Context(), slug, n, lang)
+	if err != nil {
+		s.deps.Log.Error("handleTranslationGenerate: CreateTranslationTask failed", "err", err)
+		jsonError(w, http.StatusInternalServerError, "failed to create translation task")
+		return
+	}
+
+	writeJSON(w, http.StatusAccepted, map[string]string{
+		"task_id": taskID,
+		"status":  "pending",
+		"lang":    lang,
+	})
+}
+
+// handleTranslationStatus handles GET /api/translation/status/{slug}/{n}.
+// Query params: lang (required)
+func (s *Server) handleTranslationStatus(w http.ResponseWriter, r *http.Request) {
+	if s.deps.TranslationStore == nil {
+		writeJSON(w, 0, map[string]string{"status": "unavailable"})
+		return
+	}
+
+	slug := r.PathValue("slug")
+	n, err := strconv.Atoi(r.PathValue("n"))
+	if err != nil || n < 1 || slug == "" {
+		jsonError(w, http.StatusBadRequest, "invalid params")
+		return
+	}
+
+	lang := r.URL.Query().Get("lang")
+	if !supportedTranslationLangs[lang] {
+		jsonError(w, http.StatusBadRequest, "unsupported lang")
+		return
+	}
+
+	// Fast path: translation exists in MinIO
+	key := s.deps.TranslationStore.TranslationObjectKey(lang, slug, n)
+	if s.deps.TranslationStore.TranslationExists(r.Context(), key) {
+		writeJSON(w, 0, map[string]string{"status": "done", "lang": lang})
+		return
+	}
+
+	cacheKey := fmt.Sprintf("%s/%d/%s", slug, n, lang)
+	task, found, _ := s.deps.TaskReader.GetTranslationTask(r.Context(), cacheKey)
+	if !found {
+		writeJSON(w, 0, map[string]string{"status": "idle", "lang": lang})
+		return
+	}
+
+	resp := map[string]string{
+		"status":  string(task.Status),
+		"task_id": task.ID,
+		"lang":    lang,
+	}
+	if task.Status == domain.TaskStatusFailed && task.ErrorMessage != "" {
+		resp["error"] = task.ErrorMessage
+	}
+	writeJSON(w, 0, resp)
+}
+
+// handleTranslationRead handles GET /api/translation/{slug}/{n}.
+// Query params: lang (required)
+//
+// Returns {"html": "<p>...</p>", "lang": "ru"} from the MinIO-cached translation.
+// Returns 404 when the translation has not been generated yet.
+func (s *Server) handleTranslationRead(w http.ResponseWriter, r *http.Request) {
+	if s.deps.TranslationStore == nil {
+		http.Error(w, `{"error":"machine translation not configured"}`, http.StatusServiceUnavailable)
+		return
+	}
+
+	slug := r.PathValue("slug")
+	n, err := strconv.Atoi(r.PathValue("n"))
+	if err != nil || n < 1 || slug == "" {
+		jsonError(w, http.StatusBadRequest, "invalid params")
+		return
+	}
+
+	lang := r.URL.Query().Get("lang")
+	if !supportedTranslationLangs[lang] {
+		jsonError(w, http.StatusBadRequest, "unsupported lang")
+		return
+	}
+
+	key := s.deps.TranslationStore.TranslationObjectKey(lang, slug, n)
+	md, err := s.deps.TranslationStore.GetTranslation(r.Context(), key)
+	if err != nil {
+		s.deps.Log.Warn("handleTranslationRead: translation not found", "slug", slug, "n", n, "lang", lang, "err", err)
+		jsonError(w, http.StatusNotFound, "translation not available")
+		return
+	}
+
+	var buf bytes.Buffer
+	if err := goldmark.Convert([]byte(md), &buf); err != nil {
+		s.deps.Log.Error("handleTranslationRead: markdown conversion failed", "err", err)
+		jsonError(w, http.StatusInternalServerError, "failed to render translation")
+		return
+	}
+
+	writeJSON(w, 0, map[string]string{"html": buf.String(), "lang": lang})
+}
+
+// handleAdminTranslationJobs handles GET /api/admin/translation/jobs.
+// Returns the full list of translation jobs sorted by started descending.
+func (s *Server) handleAdminTranslationJobs(w http.ResponseWriter, r *http.Request) {
+	tasks, err := s.deps.TaskReader.ListTranslationTasks(r.Context())
+	if err != nil {
+		s.deps.Log.Error("handleAdminTranslationJobs: ListTranslationTasks failed", "err", err)
+		jsonError(w, http.StatusInternalServerError, "failed to list translation jobs")
+		return
+	}
+	type jobRow struct {
+		ID           string `json:"id"`
+		CacheKey     string `json:"cache_key"`
+		Slug         string `json:"slug"`
+		Chapter      int    `json:"chapter"`
+		Lang         string `json:"lang"`
+		Status       string `json:"status"`
+		WorkerID     string `json:"worker_id"`
+		ErrorMessage string `json:"error_message"`
+		Started      string `json:"started"`
+		Finished     string `json:"finished"`
+	}
+	rows := make([]jobRow, 0, len(tasks))
+	for _, t := range tasks {
+		rows = append(rows, jobRow{
+			ID:           t.ID,
+			CacheKey:     t.CacheKey,
+			Slug:         t.Slug,
+			Chapter:      t.Chapter,
+			Lang:         t.Lang,
+			Status:       string(t.Status),
+			WorkerID:     t.WorkerID,
+			ErrorMessage: t.ErrorMessage,
+			Started:      t.Started.Format(time.RFC3339),
+			Finished:     t.Finished.Format(time.RFC3339),
+		})
+	}
+	writeJSON(w, 0, map[string]any{"jobs": rows})
+}
+
+// handleAdminTranslationBulk handles POST /api/admin/translation/bulk.
+// Body: {"slug": "...", "lang": "ru", "from": 1, "to": 50}
+// Enqueues one translation task per chapter in the range [from, to] inclusive.
+func (s *Server) handleAdminTranslationBulk(w http.ResponseWriter, r *http.Request) {
+	var body struct {
+		Slug string `json:"slug"`
+		Lang string `json:"lang"`
+		From int    `json:"from"`
+		To   int    `json:"to"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		jsonError(w, http.StatusBadRequest, "invalid JSON body")
+		return
+	}
+	if body.Slug == "" {
+		jsonError(w, http.StatusBadRequest, "slug is required")
+		return
+	}
+	if !supportedTranslationLangs[body.Lang] {
+		jsonError(w, http.StatusBadRequest, "unsupported lang; use ru, id, pt, or fr")
+		return
+	}
+	if body.From < 1 || body.To < body.From {
+		jsonError(w, http.StatusBadRequest, "from must be >= 1 and to must be >= from")
+		return
+	}
+	if body.To-body.From > 999 {
+		jsonError(w, http.StatusBadRequest, "range too large; max 1000 chapters per request")
+		return
+	}
+
+	var taskIDs []string
+	for n := body.From; n <= body.To; n++ {
+		id, err := s.deps.Producer.CreateTranslationTask(r.Context(), body.Slug, n, body.Lang)
+		if err != nil {
+			s.deps.Log.Error("handleAdminTranslationBulk: CreateTranslationTask failed",
+				"slug", body.Slug, "chapter", n, "lang", body.Lang, "err", err)
+			jsonError(w, http.StatusInternalServerError,
+				fmt.Sprintf("failed to create task for chapter %d: %s", n, err))
+			return
+		}
+		taskIDs = append(taskIDs, id)
+	}
+
+	writeJSON(w, http.StatusAccepted, map[string]any{
+		"enqueued": len(taskIDs),
+		"task_ids": taskIDs,
+	})
+}
+
+// ── Voices ─────────────────────────────────────────────────────────────────────
 // Returns {"voices": [...]} — merged list from Kokoro and pocket-tts.
 func (s *Server) handleVoices(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, 0, map[string]any{"voices": s.voices(r.Context())})

backend/internal/backend/server.go

@@ -47,6 +47,8 @@ type Dependencies struct {
 	RankingStore bookstore.RankingStore
 	// AudioStore checks audio object existence and computes MinIO keys.
 	AudioStore bookstore.AudioStore
+	// TranslationStore checks translation existence and reads/writes translated markdown.
+	TranslationStore bookstore.TranslationStore
 	// PresignStore generates short-lived MinIO URLs.
 	PresignStore bookstore.PresignStore
 	// ProgressStore reads/writes per-session reading progress.
@@ -160,6 +162,15 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("GET /api/audio/status/{slug}/{n}", s.handleAudioStatus)
 	mux.HandleFunc("GET /api/audio-proxy/{slug}/{n}", s.handleAudioProxy)
 
+	// Translation task creation (backend creates task; runner executes via LibreTranslate)
+	mux.HandleFunc("POST /api/translation/{slug}/{n}", s.handleTranslationGenerate)
+	mux.HandleFunc("GET /api/translation/status/{slug}/{n}", s.handleTranslationStatus)
+	mux.HandleFunc("GET /api/translation/{slug}/{n}", s.handleTranslationRead)
+
+	// Admin translation endpoints
+	mux.HandleFunc("GET /api/admin/translation/jobs", s.handleAdminTranslationJobs)
+	mux.HandleFunc("POST /api/admin/translation/bulk", s.handleAdminTranslationBulk)
+
 	// Voices list
 	mux.HandleFunc("GET /api/voices", s.handleVoices)