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/reset-admin.js

@@ -1,9 +1,9 @@
 const path = require('path');
-const { DatabaseSync } = require('node:sqlite');
+const Database = require('better-sqlite3');
 const bcrypt = require('bcryptjs');
 
 const dbPath = path.join(__dirname, 'data/travel.db');
-const db = new DatabaseSync(dbPath);
+const db = new Database(dbPath);
 
 const hash = bcrypt.hashSync('admin123', 10);
 const existing = db.prepare('SELECT id FROM users WHERE email = ?').get('admin@admin.com');