Phase 3.7: paid pairing flow + returning chat + extension flip

- Backend: payment_sessions + pairing_failures tables; payment.service.js
  and pairing-failure.service.js (new); rewritten pairing.service.js
  (payment-gated blast + targeted "Curhat lagi" + cancel + fallback);
  rewritten extension.service.js (data-driven auto-approve with offline
  safeguard, charge-at-approval); pricing.service.js (extension tiers
  without free trial); mitra-status.service.js (countAvailableMitras
  cached path); 60s sweeper for stale payment sessions
- Backend routes: client.payment.routes, client.mitra-availability.routes,
  internal/failed-pairings.routes; client.chat.routes rewritten for
  payment-gated start + /returning + /cancel + /fallback-to-blast;
  internal/config.routes adds 4 new keys with Valkey invalidate publish
- client_app: mitra-availability poll, payment screen + notifier, pairing
  notifier rewrite (PairingTargetedWaiting + PairingFailed states),
  targeted-waiting overlay + bestie-unavailable dialog, "Curhat lagi"
  CTA, failed-pairing terminal, extension via payment-session
- mitra_app: PairingRequestType enum, returning-chat 20s countdown
  auto-dismiss, extension card "otomatis disetujui" copy
- control_center: 4 new config rows in Settings, Failed Pairings page
  (filter + paginate + action menu), sidebar + route registered
- Test infrastructure: Vitest backend (7/7 pass), Playwright CC (4/4
  pass), Maestro mobile scaffold (CLI install pending)
- Bugs found via Playwright + fixed: LoginPage labels not associated
  with inputs (a11y); backend internal CORS missing PATCH/PUT/DELETE
  in allow-methods (silent settings breakage in browsers since Stage 4)
- Docs: phase3.7.md PRD, phase3.7-plan.md, phase3.7-questions.md (Q&A),
  phase3.7-testing.md (E2E checklist), phase3.7-test-run-2026-05-03.md
  (today's run results)

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

client_app/lib/features/chat/widgets/bestie_unavailable_dialog.dart

@@ -0,0 +1,98 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+import '../../../core/availability/mitra_availability_notifier.dart';
+import '../../../core/constants.dart';
+import '../../../core/pairing/pairing_notifier.dart';
+
+/// Shown when a "Curhat lagi" attempt against a specific bestie can't proceed
+/// — either a 409 `targeted_mitra_offline` response on the targeted POST, or
+/// one of the intermediate WS events (`returning_chat_timeout`,
+/// `returning_chat_rejected`).
+///
+/// CTAs:
+/// - "Chat dengan bestie lain" — only rendered when
+///   [mitraAvailabilityProvider] reports `available == true` at the time of
+///   build. Tapping calls [Pairing.fallbackToBlast] (reuses the same payment
+///   session — no double-charge) and closes the dialog. The caller is expected
+///   to be the searching screen, which will transition into PairingSearchingData
+///   and stay put.
+/// - "Kembali" — pops dialog and routes home. Backend has already audit-logged
+///   the targeted failure; payment session stays `confirmed` until the sweeper
+///   expires it.
+class BestieUnavailableDialog extends ConsumerWidget {
+  final String paymentSessionId;
+  final String mitraName;
+  final TopicSensitivity topicSensitivity;
+
+  const BestieUnavailableDialog({
+    super.key,
+    required this.paymentSessionId,
+    required this.mitraName,
+    required this.topicSensitivity,
+  });
+
+  /// Convenience: show this dialog and return when it closes. Per project
+  /// memory ("Riverpod ref.listen in build is unsafe"), callers should
+  /// invoke this from `ref.listenManual` callbacks in `initState`, not from
+  /// `build`.
+  static Future<void> show(
+    BuildContext context, {
+    required String paymentSessionId,
+    required String mitraName,
+    required TopicSensitivity topicSensitivity,
+  }) {
+    return showDialog<void>(
+      context: context,
+      barrierDismissible: false,
+      builder: (_) => BestieUnavailableDialog(
+        paymentSessionId: paymentSessionId,
+        mitraName: mitraName,
+        topicSensitivity: topicSensitivity,
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    // Snapshot at dialog-open time — we don't keep listening, we just check
+    // whether other bestie are around right now.
+    final availabilityAsync = ref.watch(mitraAvailabilityProvider);
+    final hasOtherAvailable = availabilityAsync.valueOrNull ?? false;
+
+    return AlertDialog(
+      title: const Text('Bestie sedang tidak online'),
+      content: Text(
+        '$mitraName sedang tidak bisa menerima chat saat ini. '
+        'Kamu bisa coba chat dengan bestie lain atau kembali ke beranda.',
+      ),
+      actions: [
+        TextButton(
+          onPressed: () {
+            // Reset pairing state and route home. Payment session stays
+            // confirmed until sweeper expires it — no extra API call needed.
+            ref.read(pairingProvider.notifier).reset();
+            Navigator.of(context).pop();
+            context.go('/home');
+          },
+          child: const Text('Kembali'),
+        ),
+        if (hasOtherAvailable)
+          ElevatedButton(
+            onPressed: () {
+              // Close the dialog first, then kick off the fallback. The
+              // searching screen will pick up the new PairingSearchingData
+              // state and render normally (no targeted overlay).
+              Navigator.of(context).pop();
+              // ignore: discarded_futures
+              ref.read(pairingProvider.notifier).fallbackToBlast(
+                    paymentSessionId: paymentSessionId,
+                    topicSensitivity: topicSensitivity,
+                  );
+            },
+            child: const Text('Chat dengan bestie lain'),
+          ),
+      ],
+    );
+  }
+}