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/runner/runner.go

@@ -29,6 +29,7 @@ import (
 	"github.com/libnovel/backend/internal/bookstore"
 	"github.com/libnovel/backend/internal/domain"
 	"github.com/libnovel/backend/internal/kokoro"
+	"github.com/libnovel/backend/internal/libretranslate"
 	"github.com/libnovel/backend/internal/meili"
 	"github.com/libnovel/backend/internal/orchestrator"
 	"github.com/libnovel/backend/internal/pockettts"
@@ -48,6 +49,8 @@ type Config struct {
 	MaxConcurrentScrape int
 	// MaxConcurrentAudio limits simultaneous audio-generation goroutines.
 	MaxConcurrentAudio int
+	// MaxConcurrentTranslation limits simultaneous translation goroutines.
+	MaxConcurrentTranslation int
 	// OrchestratorWorkers is the chapter-scraping parallelism inside each book run.
 	OrchestratorWorkers int
 	// HeartbeatInterval is how often active tasks PATCH their heartbeat_at
@@ -95,6 +98,8 @@ type Dependencies struct {
 	BookReader bookstore.BookReader
 	// AudioStore persists generated audio and checks key existence.
 	AudioStore bookstore.AudioStore
+	// TranslationStore persists translated markdown and checks key existence.
+	TranslationStore bookstore.TranslationStore
 	// CoverStore stores book cover images in MinIO.
 	CoverStore bookstore.CoverStore
 	// SearchIndex indexes books in Meilisearch after scraping.
@@ -107,6 +112,9 @@ type Dependencies struct {
 	// PocketTTS is the pocket-tts client (CPU, kyutai voices: alba, marius, etc.).
 	// If nil, pocket-tts voice tasks will fail with a clear error.
 	PocketTTS pockettts.Client
+	// LibreTranslate is the machine translation client.
+	// If nil, translation tasks will fail with a clear error.
+	LibreTranslate libretranslate.Client
 	// Log is the structured logger.
 	Log *slog.Logger
 }
@@ -137,6 +145,9 @@ func New(cfg Config, deps Dependencies) *Runner {
 	if cfg.MaxConcurrentAudio <= 0 {
 		cfg.MaxConcurrentAudio = 1
 	}
+	if cfg.MaxConcurrentTranslation <= 0 {
+		cfg.MaxConcurrentTranslation = 1
+	}
 	if cfg.WorkerID == "" {
 		cfg.WorkerID = "runner"
 	}
@@ -175,6 +186,7 @@ func (r *Runner) Run(ctx context.Context) error {
 		"mode", r.mode(),
 		"max_scrape", r.cfg.MaxConcurrentScrape,
 		"max_audio", r.cfg.MaxConcurrentAudio,
+		"max_translation", r.cfg.MaxConcurrentTranslation,
 		"catalogue_refresh_interval", r.cfg.CatalogueRefreshInterval,
 		"metrics_addr", r.cfg.MetricsAddr,
 	)
@@ -208,6 +220,7 @@ func (r *Runner) mode() string {
 func (r *Runner) runPoll(ctx context.Context) error {
 	scrapeSem := make(chan struct{}, r.cfg.MaxConcurrentScrape)
 	audioSem := make(chan struct{}, r.cfg.MaxConcurrentAudio)
+	translationSem := make(chan struct{}, r.cfg.MaxConcurrentTranslation)
 	var wg sync.WaitGroup
 
 	tick := time.NewTicker(r.cfg.PollInterval)
@@ -227,7 +240,7 @@ func (r *Runner) runPoll(ctx context.Context) error {
 
 	// Run one poll immediately on startup, then on each tick.
 	for {
-		r.poll(ctx, scrapeSem, audioSem, &wg)
+		r.poll(ctx, scrapeSem, audioSem, translationSem, &wg)
 
 		select {
 		case <-ctx.Done():
@@ -252,7 +265,7 @@ func (r *Runner) runPoll(ctx context.Context) error {
 }
 
 // poll claims all available pending tasks and dispatches them to goroutines.
-func (r *Runner) poll(ctx context.Context, scrapeSem, audioSem chan struct{}, wg *sync.WaitGroup) {
+func (r *Runner) poll(ctx context.Context, scrapeSem, audioSem, translationSem chan struct{}, wg *sync.WaitGroup) {
 	// ── Heartbeat file ────────────────────────────────────────────────────
 	// Touch /tmp/runner.alive so the Docker health check can confirm the
 	// runner is actively polling. Failure is non-fatal — just log it.
@@ -335,6 +348,39 @@ audioLoop:
 			r.runAudioTask(ctx, t)
 		}(task)
 	}
+
+	// ── Translation tasks ─────────────────────────────────────────────────
+translationLoop:
+	for {
+		if ctx.Err() != nil {
+			return
+		}
+		select {
+		case translationSem <- struct{}{}:
+			// Slot acquired — proceed to claim a task.
+		default:
+			// All slots busy; leave remaining pending tasks for next tick.
+			break translationLoop
+		}
+		task, ok, err := r.deps.Consumer.ClaimNextTranslationTask(ctx, r.cfg.WorkerID)
+		if err != nil {
+			<-translationSem
+			r.deps.Log.Error("runner: ClaimNextTranslationTask failed", "err", err)
+			break
+		}
+		if !ok {
+			<-translationSem
+			break
+		}
+		r.tasksRunning.Add(1)
+		wg.Add(1)
+		go func(t domain.TranslationTask) {
+			defer wg.Done()
+			defer func() { <-translationSem }()
+			defer r.tasksRunning.Add(-1)
+			r.runTranslationTask(ctx, t)
+		}(task)
+	}
 }
 
 // newOrchestrator builds an orchestrator with the Meilisearch post-hook wired in.

backend/internal/runner/runner_test.go

@@ -48,6 +48,10 @@ func (s *stubConsumer) ClaimNextAudioTask(_ context.Context, _ string) (domain.A
 	return t, true, nil
 }
 
+func (s *stubConsumer) ClaimNextTranslationTask(_ context.Context, _ string) (domain.TranslationTask, bool, error) {
+	return domain.TranslationTask{}, false, nil
+}
+
 func (s *stubConsumer) FinishScrapeTask(_ context.Context, id string, _ domain.ScrapeResult) error {
 	s.finished = append(s.finished, id)
 	return nil
@@ -58,6 +62,11 @@ func (s *stubConsumer) FinishAudioTask(_ context.Context, id string, _ domain.Au
 	return nil
 }
 
+func (s *stubConsumer) FinishTranslationTask(_ context.Context, id string, _ domain.TranslationResult) error {
+	s.finished = append(s.finished, id)
+	return nil
+}
+
 func (s *stubConsumer) FailTask(_ context.Context, id, _ string) error {
 	s.failCalled = append(s.failCalled, id)
 	return nil

backend/internal/runner/translation.go

@@ -0,0 +1,97 @@
+package runner
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/codes"
+
+	"github.com/libnovel/backend/internal/domain"
+)
+
+// runTranslationTask executes one machine-translation task end-to-end and
+// reports the result back to PocketBase.
+func (r *Runner) runTranslationTask(ctx context.Context, task domain.TranslationTask) {
+	ctx, span := otel.Tracer("runner").Start(ctx, "runner.translation_task")
+	defer span.End()
+	span.SetAttributes(
+		attribute.String("task.id", task.ID),
+		attribute.String("book.slug", task.Slug),
+		attribute.Int("chapter.number", task.Chapter),
+		attribute.String("translation.lang", task.Lang),
+	)
+
+	log := r.deps.Log.With("task_id", task.ID, "slug", task.Slug, "chapter", task.Chapter, "lang", task.Lang)
+	log.Info("runner: translation task starting")
+
+	// Heartbeat goroutine — keeps the task alive while translation runs.
+	hbCtx, hbCancel := context.WithCancel(ctx)
+	defer hbCancel()
+	go func() {
+		tick := time.NewTicker(r.cfg.HeartbeatInterval)
+		defer tick.Stop()
+		for {
+			select {
+			case <-hbCtx.Done():
+				return
+			case <-tick.C:
+				if err := r.deps.Consumer.HeartbeatTask(ctx, task.ID); err != nil {
+					log.Warn("runner: heartbeat failed", "err", err)
+				}
+			}
+		}
+	}()
+
+	fail := func(msg string) {
+		log.Error("runner: translation task failed", "reason", msg)
+		r.tasksFailed.Add(1)
+		span.SetStatus(codes.Error, msg)
+		result := domain.TranslationResult{ErrorMessage: msg}
+		if err := r.deps.Consumer.FinishTranslationTask(ctx, task.ID, result); err != nil {
+			log.Error("runner: FinishTranslationTask failed", "err", err)
+		}
+	}
+
+	// Guard: LibreTranslate must be configured.
+	if r.deps.LibreTranslate == nil {
+		fail("libretranslate client not configured (LIBRETRANSLATE_URL is empty)")
+		return
+	}
+
+	// 1. Read raw markdown chapter.
+	raw, err := r.deps.BookReader.ReadChapter(ctx, task.Slug, task.Chapter)
+	if err != nil {
+		fail(fmt.Sprintf("read chapter: %v", err))
+		return
+	}
+	if raw == "" {
+		fail("chapter text is empty")
+		return
+	}
+
+	// 2. Translate (chunked, concurrent).
+	translated, err := r.deps.LibreTranslate.Translate(ctx, raw, "en", task.Lang)
+	if err != nil {
+		fail(fmt.Sprintf("translate: %v", err))
+		return
+	}
+
+	// 3. Store translated markdown in MinIO.
+	key := r.deps.TranslationStore.TranslationObjectKey(task.Lang, task.Slug, task.Chapter)
+	if err := r.deps.TranslationStore.PutTranslation(ctx, key, []byte(translated)); err != nil {
+		fail(fmt.Sprintf("put translation: %v", err))
+		return
+	}
+
+	// 4. Report success.
+	r.tasksCompleted.Add(1)
+	span.SetStatus(codes.Ok, "")
+	result := domain.TranslationResult{ObjectKey: key}
+	if err := r.deps.Consumer.FinishTranslationTask(ctx, task.ID, result); err != nil {
+		log.Error("runner: FinishTranslationTask failed", "err", err)
+	}
+	log.Info("runner: translation task finished", "key", key)
+}