Phase 3.5: Mitra Chat Request History (backend route + mitra app screens)

Replaces the home-screen pending-requests banner with a "Riwayat
Permintaan" CTA that opens a list of the mitra's last 20 chat requests
(any status). Pending rows pin to the top; non-pending rows open a
read-only detail screen with a "Lihat percakapan" CTA on accepted rows.

Backend:
- New service `getRecentRequestsForMitra(mitraId, { limit })` capped at
  20, pending pinned via `(response IS NULL AND status='pending_acceptance')
  DESC`. Customer call_name returned verbatim, with `'Anonim'` only as
  null-safety fallback (no anonymity-flag masking — see project memory).
- New route `GET /api/mitra/chat-requests/recent`. Strictly per-mitra
  scoped via the existing `resolveMitra` preHandler.

Mitra app:
- New `RequestResponse` enum in core/constants.dart.
- New Riverpod notifier `requestHistoryProvider` (AsyncValue<List<...>>,
  keepAlive) — pull-to-refresh + screen-mount fetch only, no WS.
- Two new screens (history list + detail) and two new GoRoutes.
- Home screen: `_PendingRequestsBanner` removed → `_RequestHistoryButton`
  Card with red count badge. Live count comes from the existing
  chatRequestProvider so nothing changes about the WS-driven badge math.

Plan + acceptance criteria in requirement/phase3.5-plan.md. flutter
analyze clean (zero new issues). Backend smoke-tested against real DB.
Real-device E2E pending.

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

mitra_app/lib/core/constants.dart

@@ -42,6 +42,26 @@ class ExtensionStatus {
   ExtensionStatus._();
 }
 
+/// Chat request notification responses (Phase 3.5).
+/// `null` on the wire means "still pending" — no enum value.
+enum RequestResponse {
+  accepted('accepted'),
+  declined('declined'),
+  missed('missed'),
+  ignored('ignored');
+
+  final String value;
+  const RequestResponse(this.value);
+
+  static RequestResponse? fromString(String? v) {
+    if (v == null) return null;
+    for (final e in values) {
+      if (e.value == v) return e;
+    }
+    return null;
+  }
+}
+
 /// Session topic sensitivity
 enum TopicSensitivity {
   regular('regular'),