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/domain/domain.go

@@ -149,3 +149,23 @@ type AudioResult struct {
 	ObjectKey    string `json:"object_key,omitempty"`
 	ErrorMessage string `json:"error_message,omitempty"`
 }
+
+// TranslationTask represents a machine-translation job stored in PocketBase.
+type TranslationTask struct {
+	ID           string     `json:"id"`
+	CacheKey     string     `json:"cache_key"` // "{slug}/{chapter}/{lang}"
+	Slug         string     `json:"slug"`
+	Chapter      int        `json:"chapter"`
+	Lang         string     `json:"lang"`
+	WorkerID     string     `json:"worker_id,omitempty"`
+	Status       TaskStatus `json:"status"`
+	ErrorMessage string     `json:"error_message,omitempty"`
+	Started      time.Time  `json:"started"`
+	Finished     time.Time  `json:"finished,omitempty"`
+}
+
+// TranslationResult is the outcome reported by the runner after finishing a TranslationTask.
+type TranslationResult struct {
+	ObjectKey    string `json:"object_key,omitempty"`
+	ErrorMessage string `json:"error_message,omitempty"`
+}