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/api/whatsappCalls.js

@@ -10,10 +10,11 @@ class WhatsappCallsAPI extends ApiClient {
     return axios.get(`${this.url}/${callId}`);
   }
 
+  // Accept a ringing call. sdpAnswer is optional — omitted in server-relay mode
+  // where the media server handles WebRTC negotiation.
   accept(callId, sdpAnswer) {
-    return axios.post(`${this.url}/${callId}/accept`, {
-      sdp_answer: sdpAnswer,
-    });
+    const body = sdpAnswer ? { sdp_answer: sdpAnswer } : {};
+    return axios.post(`${this.url}/${callId}/accept`, body);
   }
 
   reject(callId) {
@@ -24,13 +25,47 @@ class WhatsappCallsAPI extends ApiClient {
     return axios.post(`${this.url}/${callId}/terminate`);
   }
 
+  // Initiate an outbound call. sdpOffer is optional — omitted in server-relay
+  // mode where the media server generates the SDP offer for Meta.
   initiate(conversationId, sdpOffer) {
-    return axios.post(`${this.url}/initiate`, {
-      conversation_id: conversationId,
-      sdp_offer: sdpOffer,
+    const body = { conversation_id: conversationId };
+    if (sdpOffer) body.sdp_offer = sdpOffer;
+    return axios.post(`${this.url}/initiate`, body);
+  }
+
+  // Send the agent's SDP answer for the Peer B connection (server-relay mode).
+  agentAnswer(callId, sdpAnswer) {
+    return axios.post(`${this.url}/${callId}/agent_answer`, {
+      sdp_answer: sdpAnswer,
     });
   }
 
+  // Get the current agent's active call (if any). Used for reconnection on page load.
+  active() {
+    return axios.get(`${this.url}/active`);
+  }
+
+  // Reconnect to an active call after page reload. Server creates a new Peer B
+  // and returns a fresh SDP offer via ActionCable.
+  reconnect(callId) {
+    return axios.post(`${this.url}/${callId}/reconnect`);
+  }
+
+  // Join an existing call as a supervisor (listen-only by default).
+  join(callId, role = 'listen_only') {
+    return axios.post(`${this.url}/${callId}/join`, { role });
+  }
+
+  // Play an audio file to the caller via the media server.
+  playAudio(callId, { filePath, mode = 'replace', loop = false }) {
+    return axios.post(`${this.url}/${callId}/play_audio`, {
+      file_path: filePath,
+      mode,
+      loop,
+    });
+  }
+
+  // Legacy: upload browser-side recording. Deprecated when media server is enabled.
   uploadRecording(callId, blob) {
     const formData = new FormData();
     formData.append('recording', blob, `call-${callId}.webm`);