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>

scripts/pb-init-v3.sh

@@ -294,6 +294,23 @@ create "translation_jobs" '{
     {"name":"heartbeat_at", "type":"date"}
   ]}'
 
+create "ai_jobs" '{
+  "name":"ai_jobs","type":"base","fields":[
+    {"name":"kind",          "type":"text",  "required":true},
+    {"name":"slug",          "type":"text"},
+    {"name":"status",        "type":"text",  "required":true},
+    {"name":"from_item",     "type":"number"},
+    {"name":"to_item",       "type":"number"},
+    {"name":"items_done",    "type":"number"},
+    {"name":"items_total",   "type":"number"},
+    {"name":"model",         "type":"text"},
+    {"name":"payload",       "type":"text"},
+    {"name":"error_message", "type":"text"},
+    {"name":"started",       "type":"date"},
+    {"name":"finished",      "type":"date"},
+    {"name":"heartbeat_at",  "type":"date"}
+  ]}'
+
 create "discovery_votes" '{
   "name":"discovery_votes","type":"base","fields":[
     {"name":"session_id","type":"text","required":true},