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/models.py

@@ -63,5 +63,7 @@ class TraefikGlobalStat(Base):
     )
     open_conns_total: Mapped[float | None] = mapped_column(Float, nullable=True)
     config_reloads_total: Mapped[float | None] = mapped_column(Float, nullable=True)
-    config_last_reload_success: Mapped[float | None] = mapped_column(Float, nullable=True)
+    # When Traefik last loaded its configuration successfully: its
+    # traefik_config_last_reload_success gauge is that moment, not a 0/1 flag.
+    config_last_reload_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     process_memory_bytes: Mapped[float | None] = mapped_column(Float, nullable=True)