perf: cache /measurements/latest; stale-on-error fallback for cached endpoints

The response caches (HII feeds, and now /measurements/latest at 45s TTL
— the endpoint every dashboard poll hits) share one helper,
_ttl_cached_stale: single-flight per key, empty results never cached,
and expired entries kept as a fallback. If a recompute fails (DB
unreachable), the last good response is served with an X-Data-Stale:
true header instead of a 5xx — during an outage the dashboard keeps
showing the last real readings with their honest timestamps. TTLs:
LATEST_CACHE_TTL_SECONDS (45), HII_CACHE_TTL_SECONDS (120).

src/config.py

@@ -88,6 +88,8 @@ class Config:
     HII_BASIN_CODE = int(os.getenv("HII_BASIN_CODE", "6"))  # 6 = Ping Basin
     # TTL for the /api/hii/*/latest response cache; source data changes hourly
     HII_CACHE_TTL_SECONDS = int(os.getenv("HII_CACHE_TTL_SECONDS", "120"))
+    # TTL for the /measurements/latest response cache (hottest endpoint)
+    LATEST_CACHE_TTL_SECONDS = int(os.getenv("LATEST_CACHE_TTL_SECONDS", "45"))
 
     # Umami analytics (self-hosted). The website id is public (it ships in the
     # dashboard <script> tag); server-side API tracking posts to /api/send.