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/src/scheduler.js

@@ -120,4 +120,9 @@ function startDemoReset() {
   console.log('[Demo] Hourly reset scheduled (at :00 every hour)');
 }
 
-module.exports = { start, startDemoReset, loadSettings, saveSettings, VALID_INTERVALS };
+function stop() {
+  if (currentTask) { currentTask.stop(); currentTask = null; }
+  if (demoTask) { demoTask.stop(); demoTask = null; }
+}
+
+module.exports = { start, stop, startDemoReset, loadSettings, saveSettings, VALID_INTERVALS };