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>

docker-compose.yml

@@ -8,13 +8,19 @@ services:
         condition: service_healthy
       ollama:
         condition: service_started
+    volumes:
+      - image_cache:/data/images
+      # To use a bind mount instead (gives direct host access to cached images):
+      # - ./image_cache:/data/images
     environment:
       DATABASE_URL: "postgresql+asyncpg://${POSTGRES_USER:-fabled}:${POSTGRES_PASSWORD:-fabled}@db:5432/${POSTGRES_DB:-fabledassistant}"
       OLLAMA_URL: "http://ollama:11434"
       OLLAMA_MODEL: "${OLLAMA_MODEL:-llama3.1}"
       SECRET_KEY: "${SECRET_KEY:-dev-secret-change-me}"
-      # Uncomment and set to enable web research via SearXNG:
+      # Uncomment and set to enable web research and image search via SearXNG:
       # SEARXNG_URL: "http://searxng:8080"
+      # IMAGE_CACHE_DIR: /data/images  # default, change if using a different mount path
+      # IMAGE_MAX_BYTES: "5242880"     # 5 MB per image, adjust if needed
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:5000/api/health')"]
       interval: 10s
@@ -56,3 +62,4 @@ services:
 volumes:
   pgdata:
   ollama_models:
+  image_cache: