Phase 2: Add OTel env vars and telemetry network to docker-compose

Wires the budget container to the OTel Collector via the shared external
telemetry network. Endpoint, protocol, and service name come from env vars
so the collector address is not baked into application code.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -7,6 +7,9 @@ services:
       - VIRTUAL_PORT=8080
       - ASPNETCORE_ENVIRONMENT=${ASPNETCORE_ENVIRONMENT:-Production}
       - ConnectionStrings__DefaultConnection=Host=apps-db;Port=5432;Database=${POSTGRES_DB:-budget};Username=${POSTGRES_USER:-budget};Password=${POSTGRES_PASSWORD}
+      - OTEL_EXPORTER_OTLP_ENDPOINT=http://otel-collector:4317
+      - OTEL_EXPORTER_OTLP_PROTOCOL=grpc
+      - OTEL_SERVICE_NAME=budget
     depends_on:
       - apps-db
       - auth
@@ -14,6 +17,7 @@ services:
       - web
       - apps-internal
       - auth-public
+      - telemetry
     restart: unless-stopped
 
   apps-db:
@@ -35,6 +39,8 @@ networks:
     external: true
   auth-public:
     external: true
+  telemetry:
+    external: true
 
 volumes:
   apps-db-data: