perf: 1s TTL cache for /health DB probe and /config state reads

Locust hit /health and /config on every @task(3), so each request was
firing repo.get_total_logs() and two repo.get_state() calls against
aiosqlite — filling the driver queue for data that changes on the order
of seconds, not milliseconds.

Both caches follow the shape already used by the existing Docker cache:
- asyncio.Lock with double-checked TTL so concurrent callers collapse
  into one DB hit per 1s window.
- _reset_* helpers called from tests/api/conftest.py::setup_db so the
  module-level cache can't leak across tests.

tests/test_health_config_cache.py asserts 50 concurrent callers
produce exactly 1 repo call, and the cache expires after TTL.

tests/api/conftest.py

@@ -53,6 +53,12 @@ async def setup_db(monkeypatch) -> AsyncGenerator[None, None]:
     monkeypatch.setattr(repo, "engine", engine)
     monkeypatch.setattr(repo, "session_factory", session_factory)
 
+    # Reset per-request TTL caches so they don't leak across tests
+    from decnet.web.router.health import api_get_health as _h
+    from decnet.web.router.config import api_get_config as _c
+    _h._reset_db_cache()
+    _c._reset_state_cache()
+
     # Create schema
     async with engine.begin() as conn:
         await conn.run_sync(SQLModel.metadata.create_all)