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/taskqueue/taskqueue.go

@@ -29,6 +29,10 @@ type Producer interface {
 	// returns the assigned PocketBase record ID.
 	CreateAudioTask(ctx context.Context, slug string, chapter int, voice string) (string, error)
 
+	// CreateTranslationTask inserts a new translation task with status=pending and
+	// returns the assigned PocketBase record ID.
+	CreateTranslationTask(ctx context.Context, slug string, chapter int, lang string) (string, error)
+
 	// CancelTask transitions a pending task to status=cancelled.
 	// Returns ErrNotFound if the task does not exist.
 	CancelTask(ctx context.Context, id string) error
@@ -46,13 +50,21 @@ type Consumer interface {
 	// Returns (zero, false, nil) when the queue is empty.
 	ClaimNextAudioTask(ctx context.Context, workerID string) (domain.AudioTask, bool, error)
 
+	// ClaimNextTranslationTask atomically finds the oldest pending translation task,
+	// sets its status=running and worker_id=workerID, and returns it.
+	// Returns (zero, false, nil) when the queue is empty.
+	ClaimNextTranslationTask(ctx context.Context, workerID string) (domain.TranslationTask, bool, error)
+
 	// FinishScrapeTask marks a running scrape task as done and records the result.
 	FinishScrapeTask(ctx context.Context, id string, result domain.ScrapeResult) error
 
 	// FinishAudioTask marks a running audio task as done and records the result.
 	FinishAudioTask(ctx context.Context, id string, result domain.AudioResult) error
 
-	// FailTask marks a task (scrape or audio) as failed with an error message.
+	// FinishTranslationTask marks a running translation task as done and records the result.
+	FinishTranslationTask(ctx context.Context, id string, result domain.TranslationResult) error
+
+	// FailTask marks a task (scrape, audio, or translation) as failed with an error message.
 	FailTask(ctx context.Context, id, errMsg string) error
 
 	// HeartbeatTask updates the heartbeat_at timestamp on a running task.
@@ -81,4 +93,11 @@ type Reader interface {
 	// GetAudioTask returns the most recent audio task for cacheKey.
 	// Returns (zero, false, nil) if not found.
 	GetAudioTask(ctx context.Context, cacheKey string) (domain.AudioTask, bool, error)
+
+	// ListTranslationTasks returns all translation tasks sorted by started descending.
+	ListTranslationTasks(ctx context.Context) ([]domain.TranslationTask, error)
+
+	// GetTranslationTask returns the most recent translation task for cacheKey.
+	// Returns (zero, false, nil) if not found.
+	GetTranslationTask(ctx context.Context, cacheKey string) (domain.TranslationTask, bool, error)
 }