[Feature] Add a 14d time window to surface the full free-plan retention

[maquchizi]

Problem

With METRICS_RETENTION_DAYS set (e.g. 14 on the free Grafana Cloud plan), the largest window that reports an honest summary figure is 7d. 30d and 1y correctly render — because their labels would overstate the data behind them.

The gap: the plan actually retains ~14 days, but there is no window that summarizes the 8–14 day slice. The per-window uptime/response numbers jump straight from 7d to —, so roughly the second week of retained data is never shown as a headline figure anywhere (overview list, per-site uptime grid, or the window selector).

Note this is specifically about the summary numbers. The per-site charts are not affected — fetchSiteHistory already clamps the bars, response line, and min/avg/max to retentionSec(), so selecting 30d/1y on a detail page plots the full ~14 retained days. The retained data is fetched and drawn; it just isn't quantified as a labeled percentage.

Proposed solution

Add a 14d entry to the window set so the full free-plan retention has an honest, properly-labeled summary figure.

• Extend WindowKey and WINDOWS in lib/types.ts with { key: "14d", label: "14 days", seconds: 1_209_600 }, ordered between 7d and 30d.
• Add a BAR_COUNT["14d"] in lib/buckets.ts (e.g. 84 → clean 4h buckets, since 84 × 14400 = 1_209_600), keeping the "count × step == span" invariant.
• Everything else is window-generic: coveredWindows(), windowWithinRetention(), the query fan-out, mock data, and the coverage clamp all iterate WINDOWS, so 14d flows through with no special-casing.
• Under METRICS_RETENTION_DAYS=14, 14d becomes the largest covered window, so defaultWindow() naturally opens the overview on it — the page defaults to the max data the plan retains.

Scope / things to get right

• Layout: the per-site grid is grid-cols-2 sm:grid-cols-4 and would now hold five cells — needs a responsive tweak (e.g. sm:grid-cols-5).
• Tests: window-map assertions in lib/synthetics.test.ts, the per-window step/count case in lib/buckets.test.ts, and the retention default in components/Overview.test.tsx hardcode the current four keys.
• Docs: mention 14d in docs/configuration.md alongside the retention section.

Alternatives considered

• Dynamically relabeled "13d/14d" figure over whatever's retained — most truthful, but the label churns with config and the moving edge of history and complicates every label render site.
• Show the clamped 14-day figure on the 30d card with a footnote — reintroduces the "30 days" showing 14 mislabeling the retention work removed.
• Leave as-is — data is still visible in charts, just not summarized; leaves a week of real data unquantified.

Additional context

Builds on the METRICS_RETENTION_DAYS work (#13); the clamping that makes 14d show real data already exists in fetchSiteHistory and the maxSpanSec param in lib/buckets.ts.