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/health_check.py

@@ -140,6 +140,10 @@ class APIHealthCheck(HealthCheck):
 
     def __init__(self, api_url: str, session, name: str = "api"):
         super().__init__(name)
+        # A liveness probe should fail fast: the default 30s timeout meant a
+        # slow upstream pinned executor threads for longer than the /health
+        # cache TTL, so the pool never drained under load.
+        self.timeout_seconds = 5
         self.api_url = api_url
         self.session = session