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/storage/store.go

@@ -53,6 +53,7 @@ var _ bookstore.PresignStore = (*Store)(nil)
 var _ bookstore.ProgressStore = (*Store)(nil)
 var _ bookstore.CoverStore = (*Store)(nil)
 var _ bookstore.TranslationStore = (*Store)(nil)
+var _ bookstore.AIJobStore = (*Store)(nil)
 var _ taskqueue.Producer = (*Store)(nil)
 var _ taskqueue.Consumer = (*Store)(nil)
 var _ taskqueue.Reader = (*Store)(nil)
@@ -1063,3 +1064,107 @@ func (s *Store) GetTranslation(ctx context.Context, key string) (string, error)
 	}
 	return string(data), nil
 }
+
+// ── AIJobStore ────────────────────────────────────────────────────────────────
+
+func (s *Store) CreateAIJob(ctx context.Context, job domain.AIJob) (string, error) {
+	payload := map[string]any{
+		"kind":        job.Kind,
+		"slug":        job.Slug,
+		"status":      string(job.Status),
+		"from_item":   job.FromItem,
+		"to_item":     job.ToItem,
+		"items_done":  job.ItemsDone,
+		"items_total": job.ItemsTotal,
+		"model":       job.Model,
+		"payload":     job.Payload,
+		"started":     job.Started.Format(time.RFC3339),
+	}
+	var out struct {
+		ID string `json:"id"`
+	}
+	if err := s.pb.post(ctx, "/api/collections/ai_jobs/records", payload, &out); err != nil {
+		return "", fmt.Errorf("CreateAIJob: %w", err)
+	}
+	return out.ID, nil
+}
+
+func (s *Store) GetAIJob(ctx context.Context, id string) (domain.AIJob, bool, error) {
+	var raw json.RawMessage
+	if err := s.pb.get(ctx, fmt.Sprintf("/api/collections/ai_jobs/records/%s", id), &raw); err != nil {
+		if strings.Contains(err.Error(), "404") {
+			return domain.AIJob{}, false, nil
+		}
+		return domain.AIJob{}, false, fmt.Errorf("GetAIJob: %w", err)
+	}
+	job, err := parseAIJob(raw)
+	if err != nil {
+		return domain.AIJob{}, false, err
+	}
+	return job, true, nil
+}
+
+func (s *Store) UpdateAIJob(ctx context.Context, id string, fields map[string]any) error {
+	return s.pb.patch(ctx, fmt.Sprintf("/api/collections/ai_jobs/records/%s", id), fields)
+}
+
+func (s *Store) ListAIJobs(ctx context.Context) ([]domain.AIJob, error) {
+	items, err := s.pb.listAll(ctx, "ai_jobs", "", "-started")
+	if err != nil {
+		return nil, fmt.Errorf("ListAIJobs: %w", err)
+	}
+	out := make([]domain.AIJob, 0, len(items))
+	for _, raw := range items {
+		j, err := parseAIJob(raw)
+		if err != nil {
+			continue
+		}
+		out = append(out, j)
+	}
+	return out, nil
+}
+
+func parseAIJob(raw json.RawMessage) (domain.AIJob, error) {
+	var r struct {
+		ID           string `json:"id"`
+		Kind         string `json:"kind"`
+		Slug         string `json:"slug"`
+		Status       string `json:"status"`
+		FromItem     int    `json:"from_item"`
+		ToItem       int    `json:"to_item"`
+		ItemsDone    int    `json:"items_done"`
+		ItemsTotal   int    `json:"items_total"`
+		Model        string `json:"model"`
+		Payload      string `json:"payload"`
+		ErrorMessage string `json:"error_message"`
+		Started      string `json:"started"`
+		Finished     string `json:"finished"`
+		HeartbeatAt  string `json:"heartbeat_at"`
+	}
+	if err := json.Unmarshal(raw, &r); err != nil {
+		return domain.AIJob{}, fmt.Errorf("parseAIJob: %w", err)
+	}
+	parseT := func(s string) time.Time {
+		if s == "" {
+			return time.Time{}
+		}
+		t, _ := time.Parse(time.RFC3339, s)
+		return t
+	}
+	return domain.AIJob{
+		ID:           r.ID,
+		Kind:         r.Kind,
+		Slug:         r.Slug,
+		Status:       domain.TaskStatus(r.Status),
+		FromItem:     r.FromItem,
+		ToItem:       r.ToItem,
+		ItemsDone:    r.ItemsDone,
+		ItemsTotal:   r.ItemsTotal,
+		Model:        r.Model,
+		Payload:      r.Payload,
+		ErrorMessage: r.ErrorMessage,
+		Started:      parseT(r.Started),
+		Finished:     parseT(r.Finished),
+		HeartbeatAt:  parseT(r.HeartbeatAt),
+	}, nil
+}