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-lock.json

@@ -15,7 +15,8 @@
         "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",
@@ -3806,6 +3807,21 @@
         "node": ">=10"
       }
     },
+    "node_modules/yaml": {
+      "version": "2.8.3",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.3.tgz",
+      "integrity": "sha512-AvbaCLOO2Otw/lW5bmh9d/WEdcDFdQp2Z2ZUH3pX9U2ihyUY0nvLv7J6TrWowklRGPYbB/IuIMfYgxaCPg5Bpg==",
+      "license": "ISC",
+      "bin": {
+        "yaml": "bin.mjs"
+      },
+      "engines": {
+        "node": ">= 14.6"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/eemeli"
+      }
+    },
     "node_modules/yargs": {
       "version": "17.7.2",
       "resolved": "https://registry.npmjs.org/yargs/-/yargs-17.7.2.tgz",