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>

tests/plugins/traefik/test_traefik_page.py

@@ -246,3 +246,42 @@ async def test_the_request_chart_says_its_figures_before_the_chart(app):
     assert "Requests/s (all routers)" in page and "max: 100" not in page
     assert "couldn't be drawn" in page                    # a failure says why in place of the chart
     assert "No Traefik requests recorded in the last 6h." in empty
+
+
+def test_routers_are_graded_on_their_recent_window_not_one_scrape():
+    """#5214: a 4xx share near the cutoff crossed it on one scrape and fell
+    back on the next, so problem cards came and went every poll."""
+    from plugins.traefik.routes import STATE_WINDOW_WORDS, grade_routers
+    spiky, quiet = _latest(e4=1.5), _latest(e4=1.5)
+    routers = [{"name": "spiky", "latest": spiky}, {"name": "quiet", "latest": quiet}]
+    # spiky's last 15 min average well under the cutoff; quiet has no window rows.
+    grade_routers(routers, {"spiky": NS(error_rate_4xx_pct=0.4, error_rate_5xx_pct=0.0, latency_p95_ms=20.0)}, TH)
+    assert routers[0]["state"] == "ok"
+    assert routers[1]["state"] == "warn" and not routers[1]["why"].endswith(STATE_WINDOW_WORDS)
+    held = [{"name": "held", "latest": _latest()}]                  # clean now, bad over the window
+    grade_routers(held, {"held": NS(error_rate_4xx_pct=3.0, error_rate_5xx_pct=0.0, latency_p95_ms=20.0)}, TH)
+    assert held[0]["state"] == "warn"
+    assert held[0]["why"] == "3.0% of responses are 4xx over the last 15 min"
+
+
+def test_the_last_reload_gauge_is_a_moment_not_a_flag():
+    """#5355: traefik_config_last_reload_success is a Unix time; read as a
+    0/1 flag it said "Reload failed" on every scrape."""
+    from plugins.traefik.scraper import compute_global_stats
+    at = compute_global_stats({"traefik_config_last_reload_success": {frozenset(): 1791460000.0},
+                               "traefik_config_reloads_total": {frozenset(): 343.0}}, None)
+    assert at["config_last_reload_at"] == datetime.fromtimestamp(1791460000.0, timezone.utc)
+    assert at["config_reloads_total"] == 343.0
+    never = compute_global_stats({"traefik_config_last_reload_success": {frozenset(): 0.0}}, None)
+    assert never["config_last_reload_at"] is None
+
+
+async def test_the_config_tile_says_when_it_last_reloaded(app):
+    stat = NS(config_last_reload_at=datetime.now(timezone.utc) - timedelta(minutes=3),
+              config_reloads_total=343.0, open_conns_total=None, process_memory_bytes=None)
+    async with app.test_request_context("/plugins/traefik/rows"):
+        page = await render_plugin_template(app, "traefik", "traefik/rows.html", problems=[], healthy=[],
+                                            gone=[], global_stat=stat, certs=[], old_certs=[], range_key="24h",
+                                            error_4xx=1.0, error_5xx=0.1)
+    assert "Config reloaded" in page and "3m ago" in page and "343 reloads" in page
+    assert "Reload failed" not in page