perf: inline cache fast path; cache /health checks

The on-box load test exposed the real stall: cache HITS were dispatched
through asyncio.to_thread, so under load a microsecond lookup queued
~11s behind slow work in the ~8-thread default executor (endpoints
answered inline — /forecast 4ms, /api/stats 2ms — while every to_thread
endpoint sat at p50 8-17s). Handlers now check TTL caches inline in the
async path via _cache_fresh() and only pay for a thread on a miss.

/health results are cached for HEALTH_CACHE_TTL_SECONDS (10): its
external RID-API probe plus DB query were occupying executor threads on
every hit, which is what jammed the pool in the first place.

src/config.py

@@ -91,6 +91,9 @@ class Config:
     # TTL for the /measurements/latest response cache (hottest endpoint)
     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"))
+
     # Web server worker processes. Above 1, uvicorn forks workers and a
     # localhost lock port elects a single background-collection leader.
     WEB_WORKERS = int(os.getenv("WEB_WORKERS", "2"))