feat: add Promtail, Loki, and Grafana log aggregation stack (refs #86)

- Add Promtail for Docker log scraping with container discovery
- Add Loki for log storage with 30-day retention
- Add Grafana with Loki datasource auto-provisioned
- Add IP whitelist middleware restricting Grafana to RFC1918 ranges
- Container count: 6 → 9

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config/promtail/config.yml

@@ -0,0 +1,21 @@
+server:
+  http_listen_port: 9080
+  grpc_listen_port: 0
+
+positions:
+  filename: /tmp/positions.yaml
+
+clients:
+  - url: http://mvp-loki:3100/loki/api/v1/push
+
+scrape_configs:
+  - job_name: containers
+    docker_sd_configs:
+      - host: unix:///var/run/docker.sock
+        refresh_interval: 5s
+    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'