Add YAML-based store configs with bidirectional sync

Stores can now be defined as YAML files in the stores/ directory.
On startup, YAML files are synced into the database. Changes made
via the admin UI are written back to YAML files automatically.

- Add store-sync service (load from files, export to files, write-back)
- Add /api/stores/sync and /api/stores/export endpoints
- Add Sync/Export buttons to admin UI
- Mount stores/ volume in Docker
- Include example store config template

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -5,8 +5,10 @@ services:
       - "${PORT:-3000}:3000"
     volumes:
       - ./data:/app/data
+      - ./stores:/app/stores
     environment:
       - NODE_ENV=production
       - DATABASE_PATH=/app/data/pricehunter.db
+      - STORES_DIR=/app/stores
       - PORT=3000
     restart: unless-stopped