feat: Web Push notification integration (PWA)

[only1isaac]

Summary

Implements full Web Push notification support for the Finchippay PWA, addressing all acceptance criteria from issue #377.

Changes

Frontend

• frontend/public/sw.js — push and notificationclick event handlers already present; confirmed working with new subscription flow
• frontend/lib/pushNotifications.ts (NEW) — requestPermission(), subscribeUser(publicKey), unsubscribeUser(publicKey), isSubscribed() helpers; converts VAPID key via urlBase64ToUint8Array
• frontend/components/PushNotificationPrompt.tsx (NEW) — Gentle opt-in banner shown after wallet connection; "Enable" / "Not Now" buttons; dismisses to localStorage so it doesn't re-nag
• frontend/pages/_app.tsx — Imports and renders PushNotificationPrompt in AppShellInner (below InstallBanner)
• frontend/.env.example — Added NEXT_PUBLIC_VAPID_PUBLIC_KEY

Backend

• backend/src/services/pushService.js (NEW) — web-push powered VAPID sender; addSubscription / removeSubscription / sendNotification; auto-cleans expired (HTTP 410) subscriptions
• backend/src/routes/push.js (NEW) — Three endpoints: GET /api/push/vapid-public-key, POST /api/push/subscribe, POST /api/push/unsubscribe; Zod validation + strictLimiter
• backend/src/server.js — Mounts push routes at /api/push
• backend/migrations/009_push_subscriptions.js (NEW) — Creates push_subscriptions table with public_key, endpoint, p256dh, auth, unique constraint on (public_key, endpoint)
• backend/package.json — Added web-push@^3.6.7
• backend/.env.example — Added VAPID_PUBLIC_KEY, VAPID_PRIVATE_KEY, VAPID_SUBJECT

Push Notification Triggers

• backend/src/services/tipsService.js — Fires push to creator on tip receipt
• backend/src/services/eventIndexer.js — Fires push for contract events: tip, escrow_create, escrow_claim, stream_open, stream_claim

Docs

• ENV.md — Documented all VAPID backend and frontend env vars

Tests

• backend/__tests__/pushService.test.js (NEW) — 21 passing tests covering addSubscription, removeSubscription, sendNotification (including 410 cleanup), and all three push route endpoints

Acceptance Criteria

Criteria	Status
Users can enable push notifications from a prompt in the app	✅ PushNotificationPrompt in AppShellInner
Push notifications received when app is closed	✅ Service worker push handler + VAPID backend
Notification click opens app to relevant page	✅ notificationclick navigates to data.url
Subscriptions stored server-side and cleaned up on unsubscribe	✅ push_subscriptions table + remove endpoint
VAPID keys configurable via env vars	✅ VAPID_PUBLIC/PRIVATE_KEY + VAPID_SUBJECT
CI: backend tests cover push service	✅ 21 tests passing

Testing

Generate VAPID keys:

npx web-push generate-vapid-keys

Add to backend/.env and frontend/.env.local, then connect a wallet — the notification prompt will appear.

Closes #377

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