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/integration/test_traefik_latest.py

@@ -108,3 +108,34 @@ def test_router_history_reads_every_router_in_one_query_bucketed(app):
     assert got[a][0].latency_p95_ms == 20.0
     assert [r.request_rate for r in got[b]] == [4.0]
     assert got[idle] == []                                            # its only row is before the range
+
+
+def test_recent_window_weights_shares_by_traffic_and_keeps_to_the_window(app):
+    """#5214: a router is graded on its last 15 minutes. An idle scrape's
+    100% 4xx counts for its trickle of traffic, not as a third of the
+    average; a row older than the window is not read."""
+    from plugins.traefik.models import TraefikMetric
+    from plugins.traefik.routes import recent_window
+
+    now = datetime(2097, 3, 1, 12, 0, tzinfo=timezone.utc) + timedelta(minutes=uuid.uuid4().int % 10_000)
+    tag = uuid.uuid4().hex[:8]
+    busy, empty = f"busy-{tag}@docker", f"empty-{tag}@docker"
+
+    def row(name, ago, rate, e4, p95=10.0):
+        return TraefikMetric(router_name=name, scraped_at=now - timedelta(minutes=ago), request_rate=rate,
+                             error_rate_4xx_pct=e4, error_rate_5xx_pct=0.0, latency_p50_ms=1.0,
+                             latency_p95_ms=p95, latency_p99_ms=3.0, response_bytes_rate=0.0)
+
+    async def go():
+        async with app.db_sessionmaker() as s:
+            async with s.begin():
+                s.add_all([row(busy, 1, 9.9, 0.0, 10.0), row(busy, 2, 0.1, 100.0, 30.0),
+                           row(busy, 30, 50.0, 100.0),                # outside the window
+                           row(empty, 30, 1.0, 100.0)])
+            async with s.begin():
+                return await recent_window(s, [busy, empty], now)
+
+    got = asyncio.run(go())
+    assert set(got) == {busy}                                         # nothing in empty's window
+    assert got[busy].error_rate_4xx_pct == pytest.approx(1.0)         # 0.1 × 100 over 10 req/s
+    assert got[busy].latency_p95_ms == pytest.approx(20.0)