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

docker-compose.yaml

@@ -110,6 +110,32 @@ services:
       - 1025:1025
       - 8025:8025
 
+  media-server:
+    image: chatwoot/media-server:latest
+    build:
+      context: ./enterprise/media-server
+      dockerfile: Dockerfile
+    ports:
+      - '4000:4000'
+      - '10000-10100:10000-10100/udp'
+    environment:
+      - AUTH_TOKEN=${MEDIA_SERVER_AUTH_TOKEN:-}
+      - RAILS_CALLBACK_URL=http://rails:3000
+      - RECORDINGS_DIR=/recordings
+      - LOG_LEVEL=info
+      - UDP_PORT_MIN=10000
+      - UDP_PORT_MAX=10100
+      - PUBLIC_IP=${MEDIA_SERVER_PUBLIC_IP:-}
+      - STUN_SERVERS=stun:stun.l.google.com:19302
+    volumes:
+      - media_recordings:/recordings
+    restart: unless-stopped
+    healthcheck:
+      test: ['CMD', 'wget', '--spider', '-q', 'http://localhost:4000/health']
+      interval: 10s
+      timeout: 5s
+      retries: 3
+
 volumes:
   postgres:
   redis:
@@ -117,3 +143,4 @@ volumes:
   node_modules:
   cache:
   bundle:
+  media_recordings: