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).

src/health_check.py

@@ -88,8 +88,11 @@ class DatabaseHealthCheck(HealthCheck):
             }
 
         try:
-            # Try to connect
-            if hasattr(self.db_adapter, "connect"):
+            # Connect only when there is no live engine yet: connect() re-runs
+            # the CREATE TABLE DDL suite, which is far too heavy per probe.
+            if getattr(self.db_adapter, "engine", None) is None and hasattr(
+                self.db_adapter, "connect"
+            ):
                 connected = self.db_adapter.connect()
                 if not connected:
                     return {