Add detailed access logging and Prometheus metrics

- accessLog: field filtering to drop sensitive headers (Authorization)
  while keeping useful ones (User-Agent, Content-Type, Referer)
- metrics: Prometheus endpoint with latency buckets (0.1, 0.3, 1.2, 5.0s)

Aligned with core's observability config.

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

stacks/traefik/traefik.yml

@@ -47,3 +47,22 @@ log:
 accessLog:
   filePath: /var/log/traefik_access.log
   format: json
+  fields:
+    defaultMode: keep
+    names:
+      clientUsername: drop
+    headers:
+      defaultMode: keep
+      names:
+        User-Agent: keep
+        Authorization: drop
+        Content-Type: keep
+        Referer: keep
+
+metrics:
+  prometheus:
+    buckets:
+      - 0.1
+      - 0.3
+      - 1.2
+      - 5.0