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>

plugins/traefik/scheduler.py

@@ -51,11 +51,12 @@ async def _do_scrape(app: "Quart") -> None:
     global _prev_metrics, _prev_time, _router_domains
 
     from datetime import datetime, timezone
-    from .models import TraefikCert, TraefikGlobalStat, TraefikMetric
+    from .models import TraefikCert, TraefikGlobalStat, TraefikMetric, TraefikStatusCode
     from .scraper import (
         api_base,
         compute_global_stats,
         compute_router_metrics,
+        compute_status_codes,
         extract_certs,
         fetch_metrics,
         fetch_router_domains,
@@ -81,6 +82,7 @@ async def _do_scrape(app: "Quart") -> None:
     elapsed = now - _prev_time if _prev_time else 60.0
     router_metrics = compute_router_metrics(current, _prev_metrics or None, elapsed)
     global_stats = compute_global_stats(current, _prev_metrics or None)
+    status_codes = compute_status_codes(current, _prev_metrics or None)
     cert_list = extract_certs(current)
 
     _prev_metrics = current
@@ -125,6 +127,12 @@ async def _do_scrape(app: "Quart") -> None:
                         recorded_at=scraped_at,
                     )
 
+            # ── 4xx and 5xx responses by code (#5711) ─────────────────────────
+            for router_name, codes in status_codes.items():
+                for code, count in codes.items():
+                    session.add(TraefikStatusCode(router_name=router_name, scraped_at=scraped_at,
+                                                  code=code, count=count))
+
             # ── Global stats ──────────────────────────────────────────────────
             session.add(TraefikGlobalStat(scraped_at=scraped_at, **global_stats))