feat(traefik): request chart shows the total, a time axis and a weighted 5xx share; says why if it can't draw (#5256)

#5256 "Traefik — Request chart: blank on the live dashboard; time axis and honest labels".

- "Req/s" was the average of the routers, not the total. Each scrape is now
  summed over routers, then averaged per bucket (request_history).
- The 5xx share is now weighted by each router's traffic. The axis is no
  longer pinned at 0–100 (suggestedMax 5).
- The x axis shows times, in the viewer's local time. The tooltip shows
  both series at that time.
- A verdict line comes first: now / peak req/s and the peak 5xx share. The
  figures read even if the chart doesn't.
- The blank live chart's cause is still unknown. Chart.js and
  chart-colors.js both load on the dashboard. A missing library or a thrown
  error is now written into the widget instead of leaving it empty, so the
  cause will show on the page.

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

tests/integration/test_traefik_latest.py

@@ -44,3 +44,28 @@ def test_each_router_reads_as_its_newest_row(app):
     got = asyncio.run(go())
     assert got[a].request_rate == 2.0 and got[a].scraped_at == now - timedelta(minutes=1)
     assert got[b].request_rate == 4.0
+
+
+def test_the_request_chart_sums_routers_and_weights_the_5xx_share(app):
+    """#5256: two routers in one scrape are 4 req/s in total, not an average
+    of 2; a 5xx share of 50% on the 3 req/s router is 37.5% of all traffic."""
+    from plugins.traefik.models import TraefikMetric
+    from plugins.traefik.routes import request_history
+
+    # Far ahead of every other test's rows, so only these fall in the window.
+    at = datetime(2099, 1, 1, 12, 0, tzinfo=timezone.utc) + timedelta(minutes=uuid.uuid4().int % 10_000)
+
+    def row(name, rate, e5):
+        return TraefikMetric(router_name=name, scraped_at=at, request_rate=rate, error_rate_4xx_pct=0.0,
+                             error_rate_5xx_pct=e5, latency_p50_ms=1.0, latency_p95_ms=2.0,
+                             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("quiet@docker", 1.0, 0.0), row("busy@docker", 3.0, 50.0)])
+            async with s.begin():
+                return await request_history(s, at, 60)
+
+    got = asyncio.run(go())
+    assert got == [{"at": at, "req_rate": 4.0, "err_pct": 37.5}]