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>

package.json

@@ -22,7 +22,8 @@
     "dotenv": "^16.4.7",
     "fastify": "^5.2.1",
     "p-limit": "^6.2.0",
-    "sql.js": "^1.11.0"
+    "sql.js": "^1.11.0",
+    "yaml": "^2.8.3"
   },
   "devDependencies": {
     "@types/node": "^22.10.0",