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/bookstore/bookstore.go

@@ -158,6 +158,19 @@ type CoverStore interface {
 	CoverExists(ctx context.Context, slug string) bool
 }
 
+// AIJobStore manages AI generation jobs tracked in PocketBase.
+type AIJobStore interface {
+	// CreateAIJob inserts a new ai_job record with status=running and returns its ID.
+	CreateAIJob(ctx context.Context, job domain.AIJob) (string, error)
+	// GetAIJob retrieves a single ai_job by ID.
+	// Returns (zero, false, nil) when not found.
+	GetAIJob(ctx context.Context, id string) (domain.AIJob, bool, error)
+	// UpdateAIJob patches an existing ai_job record with the given fields.
+	UpdateAIJob(ctx context.Context, id string, fields map[string]any) error
+	// ListAIJobs returns all ai_job records sorted by started descending.
+	ListAIJobs(ctx context.Context) ([]domain.AIJob, error)
+}
+
 // TranslationStore covers machine-translated chapter storage in MinIO.
 // The runner writes translations; the backend reads them.
 type TranslationStore interface {