feat: add in-app notification system with real-time delivery

Introduces a full in-app notification system with three types (simple,
boolean with server-side callbacks, navigate), three scopes (user, trip,
admin), fan-out persistence per recipient, and real-time push via
WebSocket. Includes a notification bell in the navbar, dropdown, dedicated
/notifications page, and a dev-only admin tab for testing all notification
variants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/services/inAppNotificationActions.ts

@@ -0,0 +1,22 @@
+type ActionHandler = (payload: Record<string, unknown>, respondingUserId: number) => Promise<void>;
+
+const actionRegistry = new Map<string, ActionHandler>();
+
+function registerAction(actionType: string, handler: ActionHandler): void {
+  actionRegistry.set(actionType, handler);
+}
+
+function getAction(actionType: string): ActionHandler | undefined {
+  return actionRegistry.get(actionType);
+}
+
+// Dev/test actions
+registerAction('test_approve', async () => {
+  console.log('[notifications] Test approve action executed');
+});
+
+registerAction('test_deny', async () => {
+  console.log('[notifications] Test deny action executed');
+});
+
+export { registerAction, getAction };