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/bookstore/bookstore.go

@@ -141,3 +141,19 @@ type CoverStore interface {
 	// CoverExists returns true when a cover image is stored for slug.
 	CoverExists(ctx context.Context, slug string) bool
 }
+
+// TranslationStore covers machine-translated chapter storage in MinIO.
+// The runner writes translations; the backend reads them.
+type TranslationStore interface {
+	// TranslationObjectKey returns the MinIO object key for a cached translation.
+	TranslationObjectKey(lang, slug string, n int) string
+
+	// TranslationExists returns true when the translation object is present in MinIO.
+	TranslationExists(ctx context.Context, key string) bool
+
+	// PutTranslation stores raw translated markdown under the given MinIO object key.
+	PutTranslation(ctx context.Context, key string, data []byte) error
+
+	// GetTranslation retrieves translated markdown from MinIO.
+	GetTranslation(ctx context.Context, key string) (string, error)
+}