feat(traefik): keep each router's 4xx and 5xx responses by status code (#5711)

The scraper folded traefik_service_requests_total{code} into two shares and
dropped the code, so a problem couldn't say whether it was scanners (404), a
login wall (401/403) or the app down (502/503/504). Each scrape now stores the
responses per router and code that occurred (traefik_status_codes, migration
traefik_006), pruned with the router metrics and left out of backups (System
180 ruling: Traefik history is not backed up). A problem card lists its codes
over the last 15 minutes, most first, or says none occurred, or that codes
aren't recorded yet.

A counter series new since the previous scrape now counts from 0 instead of
being skipped: Traefik creates a router's 502 series at its first 502, so a
first burst of 5xx read as 0% until it happened again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/backup/contents.py

@@ -38,6 +38,7 @@ EXCLUDED: frozenset[str] = frozenset({
     "docker_images", "docker_logs", "docker_metrics", "docker_metrics_hourly",
     "docker_swarm_nodes", "docker_swarm_services",
     "traefik_access_summaries", "traefik_certs", "traefik_global_stats", "traefik_metrics",
+    "traefik_status_codes",
     "unifi_alarms", "unifi_client_snapshots", "unifi_devices", "unifi_dpi_snapshots",
     "unifi_events", "unifi_fw_rules", "unifi_networks", "unifi_port_forwards",
     "unifi_speedtest_results", "unifi_wan_stats", "unifi_wlan_stats",