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>

.env.example

@@ -1,2 +1,3 @@
 PORT=3000
 DATABASE_PATH=./data/pricehunter.db
+STORES_DIR=./stores