perf(api): TTL-cache /stats + unfiltered pagination counts

Every /stats call ran SELECT count(*) FROM logs + SELECT count(DISTINCT
attacker_ip) FROM logs; every /logs and /attackers call ran an
unfiltered count for the paginator. At 500 concurrent users these
serialize through aiosqlite's worker threads and dominate wall time.

Cache at the router layer (repo stays dialect-agnostic):
  - /stats response: 5s TTL
  - /logs total (only when no filters): 2s TTL
  - /attackers total (only when no filters): 2s TTL

Filtered paths bypass the cache. Pattern reused from api_get_config
and api_get_health (asyncio.Lock + time.monotonic window + lazy lock).

tests/test_api_attackers.py

@@ -15,6 +15,14 @@ import pytest
 from fastapi import HTTPException
 
 from decnet.web.auth import create_access_token
+from decnet.web.router.attackers.api_get_attackers import _reset_total_cache
+
+
+@pytest.fixture(autouse=True)
+def _reset_attackers_cache():
+    _reset_total_cache()
+    yield
+    _reset_total_cache()
 
 
 # ─── Helpers ──────────────────────────────────────────────────────────────────