feat: add LGTM compose for the observability VM

WP-10(Terraform 설계서 6.10, 실험·앱 설계서 4.1). Mimir 3.2.1(monolithic, GCS
prefix blocks/ruler/alertmanager), Loki 3.7.8(tsdb v13, GCS), Tempo 3.0.3(OTLP
4317/4318, GCS), Grafana 13.2.2(데이터소스 Mimir·Loki·Tempo 프로비저닝). 오브젝트
스토리지는 VM SA의 ADC로 인증(키 파일 없음). Grafana 관리자 비밀번호는 Secret Manager
selfheal-grafana-admin에서 실행 시 취득. 알림 규칙 8건은 WP-14에서 추가.

vm/observability/compose.yaml

@@ -0,0 +1,55 @@
+# Observability VM (10.20.0.10): Mimir 9009, Loki 3100, Tempo 3200 (OTLP 4317/4318), Grafana 3000.
+# Object storage: the three GCS buckets, authenticated through the VM service account (no key files).
+services:
+  mimir:
+    image: grafana/mimir:3.2.1
+    container_name: mimir
+    restart: always
+    command: ["-config.file=/etc/mimir.yaml"]
+    volumes:
+      - ./mimir.yaml:/etc/mimir.yaml:ro
+      - mimir-data:/data
+    ports:
+      - "9009:9009"
+  loki:
+    image: grafana/loki:3.7.8
+    container_name: loki
+    restart: always
+    command: ["-config.file=/etc/loki.yaml"]
+    volumes:
+      - ./loki.yaml:/etc/loki.yaml:ro
+      - loki-data:/loki
+    ports:
+      - "3100:3100"
+  tempo:
+    image: grafana/tempo:3.0.3
+    container_name: tempo
+    restart: always
+    command: ["-config.file=/etc/tempo.yaml"]
+    volumes:
+      - ./tempo.yaml:/etc/tempo.yaml:ro
+      - tempo-data:/var/tempo
+    ports:
+      - "3200:3200"
+      - "4317:4317"
+      - "4318:4318"
+  grafana:
+    image: grafana/grafana:13.2.2
+    container_name: grafana
+    restart: always
+    environment:
+      GF_SECURITY_ADMIN_USER: admin
+      GF_SECURITY_ADMIN_PASSWORD: "${GF_SECURITY_ADMIN_PASSWORD}"
+      GF_SERVER_ROOT_URL: http://10.20.0.10:3000
+      GF_ANALYTICS_REPORTING_ENABLED: "false"
+    volumes:
+      - ./grafana/provisioning:/etc/grafana/provisioning:ro
+      - grafana-data:/var/lib/grafana
+    ports:
+      - "3000:3000"
+    depends_on: [mimir, loki, tempo]
+volumes:
+  mimir-data: {}
+  loki-data: {}
+  tempo-data: {}
+  grafana-data: {}