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/composables/useCallReconnection.js

@@ -0,0 +1,63 @@
+import { onMounted, computed } from 'vue';
+import { useWhatsappCallsStore } from 'dashboard/stores/whatsappCalls';
+import WhatsappCallsAPI from 'dashboard/api/whatsappCalls';
+
+/**
+ * Checks for an active WhatsApp call on page load and reconnects if found.
+ * This handles the server-relay scenario where the call persists on the media
+ * server even after the agent's browser reloads.
+ *
+ * NOTE: This composable intentionally does NOT call useWhatsappCallSession()
+ * to avoid creating duplicate side effects (beforeunload handlers, cleanup
+ * callbacks, timers). The WhatsappCallWidget owns the useWhatsappCallSession
+ * instance. This composable only sets store state and triggers the reconnect
+ * API call — the actual WebRTC setup happens when the ActionCable agent_offer
+ * event arrives and is handled by handleAgentOffer.
+ *
+ * Usage: call `useCallReconnection()` in the app-level layout component that
+ * mounts once on page load.
+ */
+export function useCallReconnection() {
+  const callsStore = useWhatsappCallsStore();
+
+  const isReconnecting = computed(() => callsStore.isReconnecting);
+
+  onMounted(async () => {
+    // Skip if there's already an active or incoming call in the store
+    if (callsStore.hasActiveCall || callsStore.hasIncomingCall) return;
+
+    try {
+      const { data } = await WhatsappCallsAPI.active();
+      if (!data?.call) return;
+
+      const activeCallData = data.call;
+
+      callsStore.setReconnecting(true);
+      callsStore.setActiveCall({
+        id: activeCallData.id,
+        callId: activeCallData.call_id,
+        direction: activeCallData.direction,
+        conversationId: activeCallData.conversation_id,
+        status: 'reconnecting',
+        serverRelay: true,
+        caller: activeCallData.caller,
+      });
+
+      // Set timer offset so the timer resumes from the correct elapsed time
+      if (activeCallData.elapsed_seconds) {
+        callsStore.setTimerOffset(activeCallData.elapsed_seconds);
+      }
+
+      // Tell the server to create a new Peer B and send us a fresh SDP offer.
+      // The server will broadcast whatsapp_call.agent_offer via ActionCable,
+      // which is handled by handleAgentOffer in actionCable.js.
+      await WhatsappCallsAPI.reconnect(activeCallData.id);
+    } catch {
+      // No active call or API/reconnect error — clear state and silent fail.
+      // clearActiveCall() also resets isReconnecting and callTimerOffset.
+      callsStore.clearActiveCall();
+    }
+  });
+
+  return { isReconnecting };
+}