feat: PWA Phase 2 — push notifications

Add web push notification support: push_subscriptions table (migration
0007), VAPID-based push service, subscribe/unsubscribe API routes, SW
push+notificationclick handlers, and subscribe UI on account page.
Triggers: new version uploaded (all project members) and version
approved/rejected (uploader).

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

packages/shared/src/validation/index.ts

@@ -2,3 +2,4 @@ export * from './auth.js';
 export * from './project.js';
 export * from './track.js';
 export * from './comment.js';
+export * from './push.js';

packages/shared/src/validation/push.ts

@@ -0,0 +1,10 @@
+import { z } from 'zod';
+
+export const subscribePushSchema = z.object({
+  endpoint: z.string().url(),
+  keys: z.object({
+    p256dh: z.string(),
+    auth: z.string(),
+  }),
+  userAgent: z.string().optional(),
+});