Fix OTel wiring to match working Auth implementation

Three corrections vs Auth project:
- Replace AddOtlpExporter() (traces only) with UseOtlpExporter() so both
  traces and logs are exported via OTLP
- Remove redundant .WithLogging() call; builder.Logging.AddOpenTelemetry()
  is sufficient on its own
- Pass OTEL_EXPORTER_OTLP_ENDPOINT/PROTOCOL through from host env vars
  instead of hardcoding the collector URL

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

docker-compose.yml

@@ -7,8 +7,8 @@ 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_EXPORTER_OTLP_ENDPOINT=${OTEL_EXPORTER_OTLP_ENDPOINT}
+      - OTEL_EXPORTER_OTLP_PROTOCOL=${OTEL_EXPORTER_OTLP_PROTOCOL}
       - OTEL_SERVICE_NAME=budget
     depends_on:
       - apps-db