feat(whatsapp-call): add server-side WebRTC media server for call persistence

Implements a Pion Go B2BUA media server sidecar that sits between Meta's
WhatsApp Cloud API and the agent's browser, enabling call persistence
across page reloads, server-side recording, multi-participant support,
audio injection, and AI integration readiness.

Go Media Server (enterprise/media-server/):
- Pion WebRTC v4 B2BUA with Peer A (Meta) and Peer B (Agent) connections
- Real-time OGG/Opus recording with crash recovery
- Audio bridge with multi-peer fan-out and AudioConsumer plugin interface
- Audio injection from OGG files with loop support for hold music
- Session manager with graceful shutdown and orphaned recording recovery
- HTTP API with Bearer token auth, health checks, and metrics

Rails Integration:
- Whatsapp::MediaServerClient HTTP client for Go sidecar communication
- Dual-mode CallService: legacy browser-direct and server-relay paths
- Media server callback controller for agent disconnect/recording/terminate
- CallRecordingFetchJob: downloads OGG from Go server to ActiveStorage/S3
- CallCleanupJob: sweeps stale ringing and in-progress calls
- New endpoints: active, agent_answer, reconnect, join, play_audio
- DB migration: media_session_id column with indexes

Frontend:
- Dual-mode composable auto-detecting legacy vs server-relay
- handleAgentOffer() for receiving SDP from media server
- useCallReconnection composable for page reload recovery
- Removed terminateCallOnUnload in server-relay mode
- Simplified outbound call flow in ConversationHeader
- New ActionCable event: whatsapp_call.agent_offer

Documentation:
- Server-side WebRTC architecture spec (1505 lines)
- Implementation plan with 119 trackable checklist items
- Feature spec, PR breakdown, and relay architecture docs

app/javascript/dashboard/components/widgets/WhatsappCallWidget.vue

@@ -1,9 +1,11 @@
 <script setup>
-import { watch, onUnmounted } from 'vue';
+import { watch, onUnmounted, onMounted } from 'vue';
 import { useRouter } from 'vue-router';
 import { useWhatsappCallSession } from 'dashboard/composables/useWhatsappCallSession';
 import Avatar from 'dashboard/components-next/avatar/Avatar.vue';
 import { useI18n } from 'vue-i18n';
+import { emitter } from 'shared/helpers/mitt';
+import { BUS_EVENTS } from 'shared/constants/busEvents';
 
 const { t } = useI18n();
 const router = useRouter();
@@ -16,6 +18,7 @@ const {
   isAccepting,
   isMuted,
   isOutboundRinging,
+  isReconnecting,
   callError,
   formattedCallDuration,
   acceptCall,
@@ -23,8 +26,27 @@ const {
   endActiveCall,
   toggleMute,
   dismissIncomingCall,
+  startDurationTimer,
 } = useWhatsappCallSession();
 
+// In server-relay mode, the timer starts when the Peer B WebRTC handshake
+// completes (not when the agent clicks accept). Listen for this event.
+const onAgentWebRTCConnected = () => {
+  startDurationTimer();
+};
+
+const onPermissionGranted = ({ contactName }) => {
+  emitter.emit(BUS_EVENTS.SHOW_ALERT, {
+    message: t('WHATSAPP_CALL.PERMISSION_GRANTED', { contactName }),
+    type: 'success',
+  });
+};
+
+onMounted(() => {
+  emitter.on('whatsapp_call:agent_webrtc_connected', onAgentWebRTCConnected);
+  emitter.on('whatsapp_call:permission_granted', onPermissionGranted);
+});
+
 // Auto-dismiss ringing calls after 30 seconds
 const autoRejectTimers = new Map();
 
@@ -77,6 +99,8 @@ watch(
 onUnmounted(() => {
   autoRejectTimers.forEach(timer => clearTimeout(timer));
   autoRejectTimers.clear();
+  emitter.off('whatsapp_call:agent_webrtc_connected', onAgentWebRTCConnected);
+  emitter.off('whatsapp_call:permission_granted', onPermissionGranted);
 });
 </script>
 
@@ -173,14 +197,20 @@ onUnmounted(() => {
         <p
           class="text-sm"
           :class="
-            isOutboundRinging ? 'text-n-slate-11' : 'font-mono text-n-teal-9'
+            isOutboundRinging || isReconnecting
+              ? 'text-n-slate-11'
+              : 'font-mono text-n-teal-9'
           "
         >
-          {{
-            isOutboundRinging
-              ? t('WHATSAPP_CALL.RINGING')
-              : formattedCallDuration
-          }}
+          <template v-if="isReconnecting">
+            {{ t('WHATSAPP_CALL.RECONNECTING') }}
+          </template>
+          <template v-else-if="isOutboundRinging">
+            {{ t('WHATSAPP_CALL.RINGING') }}
+          </template>
+          <template v-else>
+            {{ formattedCallDuration }}
+          </template>
         </p>
       </div>
       <div class="flex shrink-0 gap-2">