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>

src/fabledassistant/config.py

@@ -59,6 +59,11 @@ class Config:
     # SearXNG web search (external instance)
     SEARXNG_URL: str = os.environ.get("SEARXNG_URL", "")
 
+    # Image cache — images fetched from the web are stored here and served locally
+    IMAGE_CACHE_DIR: str = os.environ.get("IMAGE_CACHE_DIR", "/data/images")
+    # Maximum size of a single image to cache (default 5 MB)
+    IMAGE_MAX_BYTES: int = int(os.environ.get("IMAGE_MAX_BYTES", str(5 * 1024 * 1024)))
+
     @classmethod
     def oidc_enabled(cls) -> bool:
         return bool(cls.OIDC_ISSUER and cls.OIDC_CLIENT_ID and cls.OIDC_CLIENT_SECRET)