perf: single-flight /api/stats; fast-fail health probe

The on-box rerun after the inline fast path showed the cached endpoints
healthy (latest p50 72-160ms, HII ~100ms) but /api/stats at 81% timeouts
and /health at 33%: stats had a cache but NO single-flight, so every
concurrent miss ran the heavy whole-DB counts (~1.7M rows) in parallel,
re-jamming Postgres and the executor — which also dragged uncached
history windows into 60s timeouts. /api/stats now computes through
_ttl_cached_stale (one computation per 5min TTL, stale served on
failure). The health API probe fails fast (5s instead of 30s) and its
cache TTL rises to 30s, so a slow upstream can no longer pin executor
threads longer than the cache lifetime.

src/config.py

@@ -92,7 +92,7 @@ class Config:
     LATEST_CACHE_TTL_SECONDS = int(os.getenv("LATEST_CACHE_TTL_SECONDS", "45"))
 
     # TTL for /health check results (includes an external RID-API probe)
-    HEALTH_CACHE_TTL_SECONDS = int(os.getenv("HEALTH_CACHE_TTL_SECONDS", "10"))
+    HEALTH_CACHE_TTL_SECONDS = int(os.getenv("HEALTH_CACHE_TTL_SECONDS", "30"))
 
     # Web server worker processes. Above 1, uvicorn forks workers and a
     # localhost lock port elects a single background-collection leader.