feat(db): add lightweight migration system

Introduce server-migrations.ts with a named-migration runner that tracks
applied migrations in a _migrations table. runMigrations(db) is called at
startup before routes, so additive schema changes (ALTER TABLE, new settings)
are applied once and skipped on subsequent restarts.

Update ARCHITECTURE.md: five inline edits + new §4.4 documenting the convention.

server-migrations.ts

@@ -0,0 +1,40 @@
+import Database from 'better-sqlite3';
+
+interface Migration {
+  id: string;  // unique, immutable — format: NNNN_short_description
+  up: (db: InstanceType<typeof Database>) => void;
+}
+
+// Append only. Never reorder or remove entries — that would corrupt tracking.
+// Each `up` function receives the open DB handle inside an already-open transaction.
+const migrations: Migration[] = [
+  // Example:
+  // {
+  //   id: '0001_bookings_add_color',
+  //   up: (db) => {
+  //     db.exec(`ALTER TABLE bookings ADD COLUMN color TEXT NOT NULL DEFAULT 'blue'`);
+  //   },
+  // },
+];
+
+export function runMigrations(db: InstanceType<typeof Database>): void {
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS _migrations (
+      id         TEXT PRIMARY KEY,
+      applied_at TEXT NOT NULL DEFAULT (datetime('now'))
+    )
+  `);
+
+  const isApplied = db.prepare('SELECT 1 FROM _migrations WHERE id = ?');
+  const markApplied = db.prepare('INSERT INTO _migrations (id) VALUES (?)');
+
+  for (const migration of migrations) {
+    if (isApplied.get(migration.id)) continue;
+    console.log(`[Migrations] Applying: ${migration.id}`);
+    db.transaction(() => {
+      migration.up(db);
+      markApplied.run(migration.id);
+    })();
+    console.log(`[Migrations] Applied:  ${migration.id}`);
+  }
+}