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>

src/server/config.ts

@@ -4,5 +4,6 @@ export const config = {
   port: parseInt(process.env.PORT || '3000', 10),
   host: process.env.HOST || '0.0.0.0',
   databasePath: process.env.DATABASE_PATH || './data/pricehunter.db',
+  storesDir: process.env.STORES_DIR || './stores',
   isProduction: process.env.NODE_ENV === 'production',
 };