Add image search with local cache (Phase 23)

Images found via SearXNG are fetched server-side, stored on disk, and
served from /api/images/<id> — the user's browser never contacts the
original image host.  Original URLs are preserved for citation.

New files:
- alembic/versions/0016_add_image_cache.py  — image_cache table
- src/fabledassistant/models/image_cache.py — SQLAlchemy model
- src/fabledassistant/services/images.py    — fetch/store/serve logic
- src/fabledassistant/routes/images.py      — GET /api/images/<id>

Modified:
- config.py: IMAGE_CACHE_DIR (/data/images), IMAGE_MAX_BYTES (5 MB)
- research.py: _search_searxng_images() — SearXNG categories=images
- tools.py: _IMAGE_TOOLS def + search_images branch in execute_tool
- intent.py: search_images routing rule (explicit visual language only)
- app.py: register images_bp
- docker-compose.yml: image_cache named volume mounted at /data/images
- ToolCallCard.vue: "image_search" label

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

frontend/src/components/ToolCallCard.vue

@@ -58,6 +58,8 @@ const label = computed(() => {
       return "Research started";
     case "research_note":
       return "Research note";
+    case "image_search":
+      return "Image search";
     default:
       return "Tool call";
   }