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/test_health_config_cache.py

@@ -0,0 +1,67 @@
+"""
+TTL-cache contract: under concurrent load, N callers collapse to 1 repo hit
+per TTL window. Tests use fake repo objects — no real DB.
+"""
+import asyncio
+from unittest.mock import patch
+
+import pytest
+
+from decnet.web.router.health import api_get_health
+from decnet.web.router.config import api_get_config
+
+
+class _FakeRepo:
+    def __init__(self):
+        self.total_logs_calls = 0
+        self.state_calls = 0
+
+    async def get_total_logs(self):
+        self.total_logs_calls += 1
+        return 0
+
+    async def get_state(self, name: str):
+        self.state_calls += 1
+        return {"name": name}
+
+
+@pytest.mark.asyncio
+async def test_db_cache_collapses_concurrent_calls():
+    api_get_health._reset_db_cache()
+    fake = _FakeRepo()
+    with patch.object(api_get_health, "repo", fake):
+        results = await asyncio.gather(*[api_get_health._check_database_cached() for _ in range(50)])
+    assert all(r.status == "ok" for r in results)
+    assert fake.total_logs_calls == 1
+
+
+@pytest.mark.asyncio
+async def test_db_cache_expires_after_ttl(monkeypatch):
+    api_get_health._reset_db_cache()
+    monkeypatch.setattr(api_get_health, "_DB_CHECK_INTERVAL", 0.05)
+    fake = _FakeRepo()
+    with patch.object(api_get_health, "repo", fake):
+        await api_get_health._check_database_cached()
+        await asyncio.sleep(0.1)
+        await api_get_health._check_database_cached()
+    assert fake.total_logs_calls == 2
+
+
+@pytest.mark.asyncio
+async def test_config_state_cache_collapses_concurrent_calls():
+    api_get_config._reset_state_cache()
+    fake = _FakeRepo()
+    with patch.object(api_get_config, "repo", fake):
+        results = await asyncio.gather(*[api_get_config._get_state_cached("config_limits") for _ in range(30)])
+    assert all(r == {"name": "config_limits"} for r in results)
+    assert fake.state_calls == 1
+
+
+@pytest.mark.asyncio
+async def test_config_state_cache_per_key():
+    api_get_config._reset_state_cache()
+    fake = _FakeRepo()
+    with patch.object(api_get_config, "repo", fake):
+        await api_get_config._get_state_cached("config_limits")
+        await api_get_config._get_state_cached("config_globals")
+    assert fake.state_calls == 2