docs(mobile): document the full push-notification setup and its gotchas

Push notifications went from "wired but not deliverable" to fully working
end-to-end this session. Records the three separate one-time setup pieces
(EAS project id, FCM credentials baked at prebuild time, and the FCM V1
vs. Legacy service-account-key slot mixup that produced a confusing
"Unable to retrieve the FCM server key" error) so a future session doesn't
have to rediscover them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mobile/README.md

@@ -17,10 +17,10 @@ comments, repost, pet profile detail (view/follow/edit/followers-following),
 stories (tray/creation/viewer), video playback, search, health tracking
 (consent gate + weight/vet/vaccine logs), milestones, and message
 sending/receiving are done and verified end-to-end against the live backend.
-Push notifications are wired end-to-end but not yet deliverable — see "Push
-notifications" below. Android is the priority platform; iOS builds need a
-Mac + Apple Developer account and come later (same codebase, no separate
-source tree — see "Android-first, then iOS" below).
+Push notifications are fully working — see "Push notifications" below.
+Android is the priority platform; iOS builds need a Mac + Apple Developer
+account and come later (same codebase, no separate source tree — see
+"Android-first, then iOS" below).
 
 ## Architecture
 
@@ -206,9 +206,10 @@ separately. "Starting with Android" means:
   budgeted in `../mobil.md`) before an internal test track can be created.
 - iOS needs a Mac (for `expo run:ios` / EAS builds with credentials) and an
   Apple Developer account (99 €/year) — deferred until Android is validated.
-- EAS Build (`eas.json`, not yet added) is the recommended path for
-  producing signed Android/iOS binaries without a local Android
-  Studio/Xcode toolchain; add it when a first real build is needed.
+- EAS Build (`eas.json` — added, currently just default profiles) is the
+  recommended path for producing signed Android/iOS binaries without a
+  local Android Studio/Xcode toolchain; no EAS Build has been run yet, the
+  file only exists so far because `eas credentials` requires it.
 
 ## What's not built yet
 
@@ -229,17 +230,54 @@ emulator. Still missing, roughly in the order they'd unblock the most:
 
 - No i18n — the whole app is hardcoded German text, matching the sign-in screen
 
-### Push notifications — wired but not yet deliverable
+### Push notifications
 
 `usePushNotifications` (in `app/_layout.tsx`) requests notification
 permission and registers the resulting Expo push token with the backend
 (`owner.registerPushToken`); `src/lib/notify.ts` on the web side sends a
 best-effort push through Expo's push API alongside every in-app
-Notification it creates (follow, reaction, comment, message, ...). Minting
-a real token needs an **EAS project id** (`app.json`'s
-`extra.eas.projectId`, set once via `eas init` — free, no EAS Build
-required, but does need an Expo account login). Until that's done, the
-permission prompt shows and is granted, but `getExpoPushTokenAsync` has
-nothing to attach to and the hook no-ops with a console warning
-(`[push] No EAS project id configured...`) — confirmed via `adb logcat` on
-2026-09-06. No Owner row has an `expoPushToken` set until this is run.
+Notification it creates (follow, reaction, comment, message, ...).
+Verified end-to-end on 2026-09-06: permission → real Expo push token →
+registered in the DB → delivered to the Android emulator's notification
+tray.
+
+Getting here needed three separate one-time setup pieces, each easy to
+half-do and end up with a silent non-working state:
+
+1. **EAS project id** (`app.json`'s `extra.eas.projectId`) — via
+   `eas login` + `eas init --account pawfeed`. Without this,
+   `getExpoPushTokenAsync` has nothing to attach to and the hook no-ops
+   with `[push] No EAS project id configured...`.
+2. **FCM credentials for Android** (`app.json`'s `googleServicesFile` →
+   `google-services.json`, gitignored — redownload from the Firebase
+   Console's `org.pawfeed.app` Android app on a fresh clone). This is
+   baked into the native project at `expo prebuild` time, so adding it
+   needs a real rebuild (`expo prebuild --platform android` + `expo run:android`),
+   unlike the EAS project id which the dev client picks up from a plain
+   JS reload.
+3. **The FCM *service account key* uploaded to EAS** via
+   `eas credentials -p android` (needs `eas.json` to exist first, hence
+   that file). The credentials manager has two separate Android push
+   slots — **"FCM Legacy"** (a plain API key string, and Google shut the
+   underlying Legacy API down in 2024) and **"FCM V1"** (a Google Service
+   Account JSON key, what Firebase Console's "generate new private key"
+   actually gives you). Uploading the service-account JSON to the wrong
+   slot leaves EAS's send API returning
+   `Unable to retrieve the FCM server key for the recipient's app` even
+   though a real push token exists and permission is granted — that
+   symptom means step 3 landed in the wrong slot, not that steps 1–2 are
+   broken.
+
+Also note: `getExpoPushTokenAsync` can hang indefinitely rather than
+reject if the device can't reach Google's registration backend — the hook
+wraps it in a 15s timeout so a real failure surfaces instead of permanent
+silence.
+
+**Deploy note:** `owner.registerPushToken` is a *web* backend change.
+Since the mobile app's default `EXPO_PUBLIC_API_URL` is the live
+`pawfeed.org` deployment, this endpoint doesn't exist there until the web
+app is redeployed too (`ssh` to the NAS, `git pull`, `./docker/rolling-deploy.sh`
+— see root `CLAUDE.md`'s deployment section). This is the one case where
+a "mobile" feature session still needs a web deploy — [[project_mobile_deployment_target]]'s
+"mobile changes need no NAS deploy" is about `mobile/` code itself, not
+backend endpoints mobile happens to call.