feat(admin): live time-series trend on Metrics view (v26.05.09)

[ancongui]

Summary

Brings live time-series to the admin Metrics view. Selecting a numeric metric now opens a rolling Chart.js trend that polls the metric on the configured refresh interval (60-point window), instead of a static value snapshot.

What's new

• Live trend chart (themed via the existing charts.js Chart.js helper), polled at the admin refreshInterval.
• Value / Rate (Δ/s) toggle — downward steps are shown honestly (a real gauge decrease or a counter reset on restart), not clamped; Rate applies to gauges too, so blanket-zeroing would be wrong.
• Pause/Resume, a Current / Min / Max / Avg summary strip, and a measurement selector for multi-series (tagged) Prometheus metrics (switching reseeds the series).
• The measurements table refreshes live alongside the chart.
• Non-numeric metrics (e.g. python.version) show a snapshot + note instead of an empty chart.

Correctness / lifecycle

• Race-safe: a load-generation token drops superseded detail fetches and in-flight poll ticks, so switching metrics can't write a stale reading into the new metric's chart/buffer.
• The returned cleanup tears down the interval + destroys the chart on navigation — no dangling timers.
• Responsive: list/detail split stacks on mobile and the canvas resizes. Verified zero horizontal overflow at 390px and side-by-side at 1440px, in dark + light themes.

Bonus: cache correctness

• The SPA shell is now served with Cache-Control: no-cache so version-stamped (?v=…) asset URLs are revalidated after an upgrade. Previously a heuristically cached index.html could keep referencing the prior version's CSS/JS.

Verification

• Verified live via Playwright on a running demo: chart render + polling, Value/Rate, Pause freezing the buffer, multi-measurement selector + reseed, non-numeric handling, light theme, mobile (no overflow, canvas resize), navigation cleanup (poll stops, canvas removed), and race-safe rapid switching.
• Reviewed with a multi-agent adversarial workflow (4 dimensions, findings independently verified); the two confirmed findings — a poll-tick race and Rate-mode counter resets — are fixed/addressed in this PR.

Tests & gates

• 2 new SPA-shell tests (no-cache header + ?v= version stamp).
• ruff check ✓, ruff format --check ✓, mypy --strict (577 files) ✓, pytest tests/admin tests/web → 352 passed.

Version

Bumps to v26.05.09 (pyproject 26.5.9, __version__ = "26.05.09", uv.lock). CHANGELOG + admin docs updated.