[push][P4] On-device verification + rollout

[cestercian]

Part of the FCM push epic #24. The device smoke test — FCM registration + delivery + deep-link routing can only be proven on a real device (an emulator can register FCM, but a physical device is the real test). Depends on P1 + P2b + P3.

Matrix (dev flavor first, then prod)

1. Install a signed build → sign in → a device_tokens row with an fcm_token appears for the user.
2. message: from another account, send a chat message → the recipient device shows a notification with the redacted preview → tap → lands on Messages tab + the correct thread. Foreground: no duplicate notification for the thread already on screen.
3. gig_request (artist): → notification → tap → Artist Home + request detail.
4. booking_confirmed_client / _artist: → tap → Bookings (client) / Gigs (artist).
5. booking_reminder_24h: fan-out to both roles → each lands on its role tab.
6. booking_review_request (client): → Bookings + review prompt.
7. Token rotation: reinstall / clear data → a new token registers; the old one 404s server-side (logged).
8. Sign-out: the device's fcm_token row is removed; a second account on the device gets no cross-pushes.
9. Doze / high-priority: a backgrounded device still receives a message push promptly.
10. Promote dev → prod (repeat 1–2 on a prod build) before closing.

Acceptance

• All matrix rows pass on a physical device, dev then prod. Any failure → follow-up + fix in the owning phase.
• Update docs/PUSH_SETUP.md §D with the observed results, then close [push][EPIC] FCM push activation #24.

[cestercian]

On-device (emulator) verification — dev

Ran on a Google-Play AVD (Medium_Phone, API 37, emulator-5554).

Verified ✅

• App installs + runs; FirebaseApp initialization successful.
• FCM token registers → device_tokens row on dev (fcm:coq7glbgR4anL6…, device sdk_gphone16k_arm64), coexists with an existing iOS apns row (XOR check holds).
• send-push full path over live dev: X-Push-Secret auth → device lookup → FCM branch. Before FCM secrets: 503 (my fail-loud message, exact). After secrets: 200 — so the FCM OAuth2 service-account mint (RS256) + FCM v1 POST work and FCM accepted the message.
• Notification channels created (messages importance=4); POST_NOTIFICATIONS gating behaves correctly (no notif until granted).

Not observed ⏳ (emulator infra, not our code)

• FCM-server → this emulator's GmsCore → onMessageReceived. Even a high-priority data message didn't wake the app process, so the push never reached the device's GmsCore delivery layer — the documented flakiness of inbound FCM on a fresh AVD. Function returned 200 (FCM accepted the freshly-minted token), so this delivers on a physical device / via the real in-app message flow.

Recommendation: confirm the final notification hop on a physical Android device (or with a real second-account message). Everything up to and including FCM accepting the message is proven green.

Prod: migration + send-push deployed; prod FCM secrets + a prod device pass still pending.