perf: stale-while-revalidate caching, bigger executor, cheap DB health probe

Third on-box run: p50s healthy everywhere but tails at 60s — when a TTL
expired under load, every concurrent miss parked an executor thread on
the single-flight lock, exhausting the ~12-thread pool and timing out
unrelated endpoints. All cached endpoints (latest, HII, stats, health)
now use _cached_swr: fresh -> inline; expired-but-present -> the stale
value is returned immediately and ONE background task refreshes; only a
cold key (first request since startup) waits. Plus: dedicated
ThreadPoolExecutor (EXECUTOR_THREADS, default 48) replaces the cpu+4
default, and DatabaseHealthCheck no longer re-runs the CREATE TABLE DDL
suite on every probe (connect only when no live engine).

tests/test_hii_collector.py

@@ -296,6 +296,7 @@ class TestHiiApiEndpoints:
         assert collector.store.save_waterlevel(wl) == 2
         monkeypatch.setitem(web_api.app_state, "hii_collector", collector)
         web_api.HII_CACHE.clear()  # response cache would leak across tests
+        web_api._REFRESH_IN_FLIGHT.clear()
         return web_api
 
     @staticmethod
@@ -331,6 +332,7 @@ class TestHiiApiEndpoints:
 
         monkeypatch.setitem(web_api.app_state, "hii_collector", None)
         web_api.HII_CACHE.clear()
+        web_api._REFRESH_IN_FLIGHT.clear()
         assert self._get(web_api, "get_hii_rainfall_latest", hours=26)[0] == []
         assert self._get(web_api, "get_hii_waterlevel_latest", hours=26)[0] == []
 
@@ -378,6 +380,7 @@ class TestHiiApiEndpoints:
         )
         monkeypatch.setitem(web_api.app_state, "hii_collector", collector)
         web_api.HII_CACHE.clear()
+        web_api._REFRESH_IN_FLIGHT.clear()
         assert self._get(web_api, "get_hii_rainfall_latest", hours=26)[0] == []
         assert web_api.HII_CACHE == {}  # empty response left uncached
 
@@ -406,6 +409,7 @@ class TestHiiApiEndpoints:
         scraper = SimpleNamespace(db_adapter=True, get_latest_data=get_latest_data)
         monkeypatch.setitem(web_api.app_state, "scraper", scraper)
         web_api.LATEST_CACHE.clear()
+        web_api._REFRESH_IN_FLIGHT.clear()
 
         rows, response = self._get(web_api, "get_latest_measurements", limit=500)
         rows2, _ = self._get(web_api, "get_latest_measurements", limit=500)