perf(api): cache /api/observers/metrics/summary server-side (0.9 s warm, 2.7–4 s cold; now used by three views) #506

Follow-up from the #498 review (F2).

GET /api/observers/metrics/summary?window=1h takes about 0.9 s warm and 2.7–4 s cold on staging, and about 2.8–3.2 s cold on production. Since #481 and #498 it is used by:

• the observer list;
• the RF Health tiles;
• the observer detail page (which no longer blocks on it).

Proposal:

• Add a short server-side cache (for example 30 s, keyed by window) with singleflight, so concurrent misses do not stampede.
• Keep the cache bounded and read-only.
• Optionally use stale-while-revalidate.
• Measure cold and warm latency before and after on staging.