Stabilize core: better-sqlite3, versioned migrations, graceful shutdown

- Replace experimental node:sqlite with better-sqlite3 (stable API)
- Replace try/catch migration pattern with schema_version tracking
- Add SIGTERM/SIGINT handler for clean shutdown (DB flush, scheduler stop)
- Fix BudgetPanel crash: remove undefined setShowAddCategory call
- Update Dockerfile: remove --experimental-sqlite, add native build tools

server/package.json

@@ -3,12 +3,13 @@
   "version": "2.5.2",
   "main": "src/index.js",
   "scripts": {
-    "start": "node --experimental-sqlite src/index.js",
+    "start": "node src/index.js",
     "dev": "nodemon src/index.js"
   },
   "dependencies": {
     "archiver": "^6.0.1",
     "bcryptjs": "^2.4.3",
+    "better-sqlite3": "^12.8.0",
     "cors": "^2.8.5",
     "dotenv": "^16.4.1",
     "express": "^4.18.3",