fix(traefik): grade routers on their last 15 minutes; the Config tile reads Traefik's reload time (#5355)

The operator: problem cards "can disappear so quickly". Each router was
graded on its latest scrape, so a 4xx share near the 1% cutoff crossed it
on one scrape and fell back on the next. recent_window reads every
router's last 15 minutes in one query, the error shares weighted by
traffic; grade_routers uses it on the page and the dashboard widget, and
the why says "over the last 15 min".

traefik_config_last_reload_success is the Unix time of the last good
reload, not a 0/1 flag, so the tile said "Reload failed" on every scrape.
traefik_005 stores it as config_last_reload_at, and the tile reads
"Config reloaded 3m ago". The router index also carries the 4xx share.

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

plugins/traefik/scraper.py

@@ -152,21 +152,21 @@ def compute_router_metrics(
 def compute_global_stats(
     current: ParsedMetrics,
     previous: ParsedMetrics | None,
-) -> dict[str, float | None]:
+) -> dict[str, float | datetime | None]:
     """Extract process-level and config health metrics from a scrape.
 
     Returns:
         {
             "open_conns_total": float | None,
             "config_reloads_total": float | None,
-            "config_last_reload_success": float | None,   # 1.0 = success, 0.0 = failure
+            "config_last_reload_at": datetime | None,     # last successful reload
             "process_memory_bytes": float | None,
         }
     """
-    stats: dict[str, float | None] = {
+    stats: dict[str, float | datetime | None] = {
         "open_conns_total": None,
         "config_reloads_total": None,
-        "config_last_reload_success": None,
+        "config_last_reload_at": None,
         "process_memory_bytes": None,
     }
 
@@ -180,11 +180,15 @@ def compute_global_stats(
     if reloads:
         stats["config_reloads_total"] = sum(reloads.values())
 
-    # Last reload success gauge
+    # The moment of the last successful reload, as a Unix timestamp: Traefik
+    # names it "last_reload_success", but it is a time, not a 0/1 flag. Read
+    # as a flag, every scrape said "Reload failed" (#5355).
     last_success = current.get("traefik_config_last_reload_success", {})
     if last_success:
         # Usually a single sample with no labels
-        stats["config_last_reload_success"] = next(iter(last_success.values()))
+        ts = next(iter(last_success.values()))
+        if ts > 0:
+            stats["config_last_reload_at"] = datetime.fromtimestamp(ts, timezone.utc)
 
     # Process RSS memory
     mem = current.get("process_resident_memory_bytes", {})