Phase 4 §1/§5: notif banner detection on API <33 + chat-delivery WS→FCM lifecycle

§1 notif banner: permission_handler v11 returns granted unconditionally
for Permission.notification on Android <13 because POST_NOTIFICATIONS
didn't exist as a runtime permission. Result: SHome1st amber "notifikasi
off" banner never showed on API 24-32 even when the user toggled
notifications off in Settings → Apps. Add a
NotificationManagerCompat.areNotificationsEnabled() pre-check via
flutter_local_notifications (works from API 19+) so the banner reflects
the real OS state on older Android.

§5 chat delivery: the contract is "WS when foreground, FCM when
background", but the previous build only honoured (1) — Android keeps
the TCP socket alive after the Dart isolate is paused, so backend's
`socket.readyState === 1` check returned true and FCM never fired.
Fix has five parts (all required together):

 1. Customer-side lifecycle observer in client_app/main.dart closes
    chatProvider's WS on paused/detached, reconnects on resumed.
 2. `_appPaused` gate in main.dart suppresses the activeSessionProvider
    listener's auto-reconnect (15s poll in active_session_notifier
    would otherwise re-open the WS the next tick after the observer
    closed it — defeating the fallback).
 3. Mitra-side lifecycle observer in mitra_app/main.dart stashes
    `_pausedChatSessionId`, calls mitraChatProvider.disconnect(), and
    re-issues connect(saved) on resumed.
 4. MitraChat gains a `_connectedSessionId` field + getter so the
    observer in step 3 can read it back across disconnect (disconnect
    clears it; the next connect overwrites it).
 5. SearchingScreen resets pairingProvider when entering with a new
    draft.paymentId — previously it retained PairingActiveData with
    the *old* sessionId after a session ended, and the next pairing
    flow navigated straight to that completed session showing
    "Sesi sudah berakhir".

Backend additions under /internal/_test/* for assertion harness:
inspectSessionWsState + GET /ws-connection-state,
POST /send-chat-message-as-mitra (with delivered_via),
POST /send-chat-message-as-customer (with delivered_via),
POST /send-fcm-chat-message (raw FCM dispatch).

Maestro coverage:
 - ts-customer-05-01: mitra → customer message when customer is
   backgrounded → delivered_via=fcm.
 - ts-customer-05-02: customer → mitra message when mitra is
   backgrounded → delivered_via=fcm.
 - ts-customer-01-01: §1 notif-denied banner on home. Documented
   precondition: mitra must be force-stopped or backgrounded on the
   chat screen before 05-02 runs (Maestro can only drive one --udid
   per run; mitra-side lifecycle observer end-to-end is deferred).

Helper scripts under client_app/.maestro/scripts/:
inspect_ws_state.js, assert_ws_state.js,
send_chat_message_as_mitra.js, assert_delivered_via.js (takes
SENDER=mitra|customer to route to the matching backend endpoint).

README_section_05.md documents the test plan, helper scripts, and the
deferred mitra-side maestro driving. Both apps tested manually on
API 28 AVDs where FCM delivery is sub-second; API 24 has 5-30 min
heartbeats that make it impractical for FCM-related testing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/.maestro/flows/README_section_05.md

@@ -0,0 +1,93 @@
+# §5 — Chat Room test plan
+
+Spec: [requirement/flow_customer.mermaid.md §5](../../../requirement/flow_customer.mermaid.md)
+plus the user-stated message-delivery contract:
+
+> 1. WebSocket when app is foreground / WS connected.
+> 2. FCM when app is background / WS disconnected.
+
+The lifecycle observers that enforce (2) live at:
+- Customer side: `client_app/lib/main.dart::_AppState.didChangeAppLifecycleState`
+- Mitra side: `mitra_app/lib/main.dart::_AppState.didChangeAppLifecycleState` +
+  `mitra_app/lib/core/chat/mitra_chat_notifier.dart` (`_connectedSessionId`
+  + getter for the observer to read on `paused`).
+
+A 15s `Timer.periodic` in `active_session_notifier.dart` would otherwise
+re-open the customer WS right after the observer closed it; the
+`_appPaused` gate in customer main.dart suppresses that race.
+
+## Implemented
+
+| File | Direction | Expected dispatch |
+|---|---|---|
+| `ts-customer-05-01-background_disconnects_ws_so_messages_fall_back_to_fcm.yaml` | mitra → customer, customer is backgrounded | `delivered_via=fcm` |
+| `ts-customer-05-02-customer_message_to_backgrounded_mitra_falls_back_to_fcm.yaml` | customer → mitra, mitra is backgrounded | `delivered_via=fcm` |
+
+Both flows drive the customer device only (Maestro can only target one
+`--udid` per run). The mitra side is verified server-side via
+[`/internal/_test/ws-connection-state`](../../../backend/src/routes/internal/_test.routes.js)
+and
+[`/internal/_test/send-chat-message-as-customer`](../../../backend/src/routes/internal/_test.routes.js)
+(introduced 2026-05-18 with the lifecycle fix).
+
+## Helper scripts (under `../scripts/`)
+
+| Script | Purpose |
+|---|---|
+| `assert_ws_state.js` | Reads `/internal/_test/ws-connection-state?session_id=…`; throws on mismatch with `EXPECTED_CUSTOMER_CONNECTED` / `EXPECTED_MITRA_CONNECTED`. |
+| `assert_delivered_via.js` | Sends a chat message via the test endpoint matching `SENDER` (`mitra` default, or `customer`); asserts the response's `delivered_via` equals `EXPECTED_DELIVERED_VIA`. |
+| `inspect_ws_state.js` | Same data as `assert_ws_state.js` but exports the booleans into `output.CUSTOMER_CONNECTED` / `output.MITRA_CONNECTED` for downstream conditions without throwing. |
+| `send_chat_message_as_mitra.js` | Mitra→customer one-shot send. Captures `output.MESSAGE_ID` + `output.DELIVERED_VIA`. |
+
+## Pre-reqs for both flows
+
+- Backend reachable; `NODE_ENV != 'production'`.
+- ≥1 mitra online (the `seed_history_session.js` helper picks the
+  most-recently-online mitra to act as the eventual acceptor).
+- For 05-01: customer device is the one Maestro drives; `pressKey: HOME`
+  inside the flow backgrounds it. No mitra-app interaction required.
+- For 05-02: **the mitra app on `emulator-5556` must be backgrounded or
+  force-stopped before the flow runs**. The `assert_ws_state.js` guard
+  fails loudly if mitra is foreground (proving WS-over-FCM precedence —
+  also a correct test outcome, just the wrong side of the matrix). To
+  ensure FCM actually arrives on the device, the mitra account on
+  `emulator-5556` must have a `fcm_token` in `mitras.fcm_token` (i.e.,
+  the app has signed in and called `/api/shared/device-token` at some
+  point) — verified by `seed_history_session.js`'s output mitra ID and
+  the `MITRA_NAME_RE` it captures.
+
+## Manual mitra-side verification
+
+After 05-02 passes, optionally inspect the mitra device's notification
+shade to confirm the FCM tray notification rendered:
+
+```bash
+export ADB_SERVER_SOCKET=tcp:172.22.240.1:5037
+adb -s emulator-5556 shell cmd statusbar expand-notifications
+adb -s emulator-5556 exec-out screencap -p > /tmp/mitra_shade.png
+adb -s emulator-5556 shell cmd statusbar collapse
+```
+
+Expect to see a `Pesan baru dari Customer` notification within ~1s on an
+API 28+ AVD (older AVDs queue FCM for 5-30 min — see the per-AVD
+Play-Services notes in the project memory).
+
+## Not yet automated
+
+**Mitra-side lifecycle observer** end-to-end. To verify that the mitra
+app's `didChangeAppLifecycleState(paused)` actually closes its chat WS,
+the test would need to drive the mitra emulator (Maestro on
+`--udid emulator-5556`) through login → chat-screen → `pressKey: HOME` →
+assert backend WS state. This requires:
+
+1. Pre-seeding an active chat session for the currently-signed-in mitra
+   (no existing test endpoint for "seed customer + pair + active
+   session"; the existing scripts assume the customer app drives it).
+2. Driving the mitra OTP login + chat-request acceptance UI from
+   Maestro.
+
+Both are feasible but out of scope for the current iteration. The
+mitra-side fix is covered indirectly: ts-customer-05-02 asserts the
+backend correctly dispatches via FCM whenever the mitra WS is closed,
+and the mitra app's lifecycle observer was verified manually
+(2026-05-18) to be the mechanism that closes that WS.