devops(observability): add Loki + Promtail for centralised container log aggregation

- Add obs-loki (grafana/loki:3.4.2) to docker-compose.observability.yml
  with healthcheck (wget /ready), expose-only port 3100, named volume loki_data
- Add obs-promtail (grafana/promtail:3.4.2) bridging archiv-net + obs-net,
  depends_on loki service_healthy, docker.sock:ro, promtail_positions volume
  for restart-safe position tracking
- Create infra/observability/loki/loki-config.yml: single-node TSDB schema v13,
  30-day retention, auth disabled (obs-net only), telemetry off
- Create infra/observability/promtail/promtail-config.yml: Docker SD scrape,
  container_name / compose_service / compose_project / logstream labels
- Update docs/DEPLOYMENT.md §4 with service table and Loki quick-check commands

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

infra/observability/loki/loki-config.yml

@@ -0,0 +1,32 @@
+auth_enabled: false  # safe — loki is not exposed beyond obs-net. Add auth before binding port 3100 to host.
+
+server:
+  http_listen_port: 3100
+
+common:
+  instance_addr: 127.0.0.1
+  path_prefix: /loki
+  storage:
+    filesystem:
+      chunks_directory: /loki/chunks
+      rules_directory: /loki/rules
+  replication_factor: 1
+  ring:
+    kvstore:
+      store: inmemory  # correct for single-node — no etcd/consul needed here
+
+schema_config:
+  configs:
+    - from: 2024-01-01
+      store: tsdb
+      object_store: filesystem
+      schema: v13
+      index:
+        prefix: index_
+        period: 24h
+
+limits_config:
+  retention_period: 720h  # 30 days — low-volume family archive; revisit if log volume grows
+
+analytics:
+  reporting_enabled: false  # no telemetry sent to Grafana Labs