feat: AI job tracking, range support, auto-prompt, and resume

- New `ai_jobs` PocketBase collection tracks all long-running AI tasks
  (batch-covers, chapter-names) with status, progress, and cancellation
- `handlers_aijobs.go`: GET/cancel endpoints for ai_jobs; centralised
  cancel registry (moved from handlers_catalogue)
- Batch-covers and chapter-names SSE handlers now create/resume ai_job
  records, support from_item/to_item ranges, and resume from items_done
  on restart via job_id
- New `POST /api/admin/image-gen/auto-prompt`: generates an image prompt
  from book description (cover) or chapter title (chapter) via LLM
- image-gen page: "Auto-prompt" button calls auto-prompt API when a slug
  is selected; falls back gracefully if TextGen not configured
- text-gen chapter-names: from/to chapter range inputs + job ID display
- catalogue-tools batch-covers: from/to item range + resume job ID input
- pb-init-v3.sh: adds ai_jobs collection (idempotent)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/domain/domain.go

@@ -169,3 +169,30 @@ type TranslationResult struct {
 	ObjectKey    string `json:"object_key,omitempty"`
 	ErrorMessage string `json:"error_message,omitempty"`
 }
+
+// AIJob represents an AI generation task tracked in PocketBase (ai_jobs collection).
+type AIJob struct {
+	ID string `json:"id"`
+	// Kind is one of: "chapter-names", "batch-covers", "chapter-covers", "refresh-metadata".
+	Kind string `json:"kind"`
+	// Slug is the book slug for per-book jobs; empty for catalogue-wide jobs.
+	Slug   string     `json:"slug"`
+	Status TaskStatus `json:"status"`
+	// FromItem is the first item to process (chapter number, or 0-based book index).
+	// 0 = start from the beginning.
+	FromItem int `json:"from_item"`
+	// ToItem is the last item to process (inclusive). 0 = process all.
+	ToItem int `json:"to_item"`
+	// ItemsDone is the cumulative count of successfully processed items.
+	ItemsDone int `json:"items_done"`
+	// ItemsTotal is the total number of items in this job.
+	ItemsTotal int `json:"items_total"`
+	Model      string `json:"model"`
+	// Payload is a JSON-encoded string with job-specific parameters
+	// (e.g. naming pattern for chapter-names, num_steps for batch-covers).
+	Payload      string    `json:"payload"`
+	ErrorMessage string    `json:"error_message,omitempty"`
+	Started      time.Time `json:"started,omitempty"`
+	Finished     time.Time `json:"finished,omitempty"`
+	HeartbeatAt  time.Time `json:"heartbeat_at,omitempty"`
+}