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/routes/images.py

@@ -0,0 +1,35 @@
+"""Serve locally-cached images.
+
+No authentication required — image IDs are opaque and unguessable (based
+on the SHA-256 of the original URL).  Images are served with a 1-day
+Cache-Control header so the browser doesn't re-request on every page load.
+"""
+
+import logging
+
+from quart import Blueprint, jsonify, send_file
+
+from fabledassistant.services.images import get_image_path, get_image_record
+
+logger = logging.getLogger(__name__)
+
+images_bp = Blueprint("images", __name__, url_prefix="/api/images")
+
+
+@images_bp.route("/<int:image_id>", methods=["GET"])
+async def serve_image(image_id: int):
+    """Serve a locally-cached image by its DB ID."""
+    record = await get_image_record(image_id)
+    if record is None:
+        return jsonify({"error": "Image not found"}), 404
+
+    file_path = get_image_path(record)
+    if not file_path.exists():
+        logger.warning("Image file missing on disk for id=%d (%s)", image_id, file_path)
+        return jsonify({"error": "Image file not found"}), 404
+
+    return await send_file(
+        file_path,
+        mimetype=record.content_type,
+        max_age=86400,
+    )