feat(06-02): add Loki + Promtail + Grafana stack to docker-compose

- Create docker/loki/loki-config.yaml: single-binary filesystem-mode Loki
  (schema v13, tsdb store, embedded_cache 100MB, auth_enabled: false)
- Create docker/loki/promtail-config.yaml: docker_sd_configs scrape with
  label filter logging=promtail; relabels container name and service labels
- docker-compose.yml: add loki (:3100), promtail, grafana (:3000) services
  after celery-beat; loki_data and grafana_data named volumes; GF_AUTH
  anonymous enabled for local dev; backend + celery-worker get logging:
  "promtail" label; backend gets LOG_LEVEL/LOG_JSON env vars
- celery-beat deliberately left without logging: label (not a network-facing
  service; schedule file write concerns deferred per D-08 Pitfall 7 notes)

docker/loki/promtail-config.yaml

@@ -0,0 +1,24 @@
+server:
+  http_listen_port: 9080
+  grpc_listen_port: 0
+
+positions:
+  filename: /tmp/positions.yaml
+
+clients:
+  - url: http://loki:3100/loki/api/v1/push
+
+scrape_configs:
+  - job_name: docker
+    docker_sd_configs:
+      - host: unix:///var/run/docker.sock
+        refresh_interval: 5s
+        filters:
+          - name: label
+            values: ["logging=promtail"]
+    relabel_configs:
+      - source_labels: ["__meta_docker_container_name"]
+        regex: "/(.*)"
+        target_label: "container"
+      - source_labels: ["__meta_docker_container_label_com_docker_compose_service"]
+        target_label: "service"