fix: защита от дубликатов сканов — UPDATED-only + per-URL мьютекс

- constants.py: RELEVANT_WEBHOOK_ACTIONS теперь только UPDATED
  (CREATED игнорируется, Nexs proxy шлёт UPDATED при обновл кэша)
- harvester.py: asyncio.Lock на каждый download_url
  — при параллельных вебхуках только первый пройдёт, остальные skipped
  — lock проверяется + DB re-check внутри критической секции
- tests: обновлены фикстуры (CREATED→UPDATED), добавлен тест ignores_created

tests/conftest.py

@@ -99,7 +99,7 @@ def sample_nexus_webhook():
         "timestamp": "2026-05-09T12:00:00.000+00:00",
         "nodeId": "test-node",
         "initiator": "admin",
-        "action": "CREATED",
+        "action": "UPDATED",
         "repositoryName": "pypi-proxy",
         "asset": {
             "id": "abc123",
@@ -117,7 +117,7 @@ def sample_nexus_component_webhook():
         "timestamp": "2026-05-09T12:00:00.000+00:00",
         "nodeId": "test-node",
         "initiator": "admin",
-        "action": "CREATED",
+        "action": "UPDATED",
         "repositoryName": "pypi-proxy",
         "component": {
             "id": "comp1",

tests/test_webhooks.py

@@ -24,14 +24,11 @@ async def test_webhook_ignores_deleted_action(client, sample_nexus_webhook):
 
 
 @pytest.mark.asyncio
-async def test_webhook_accepts_asset_created(client, sample_nexus_webhook):
-    with patch("guarddog_nexus.webhooks._scan_in_background") as _mock:
-        resp = await client.post("/webhooks/nexus", json=sample_nexus_webhook)
-        assert resp.status_code == 200
-        data = resp.json()
-        assert data["status"] == "accepted"
-        assert data["action"] == "CREATED"
-        assert "/packages/requests/2.31.0/requests-2.31.0.tar.gz" in data["asset"]
+async def test_webhook_ignores_created_action(client, sample_nexus_webhook):
+    sample_nexus_webhook["action"] = "CREATED"
+    resp = await client.post("/webhooks/nexus", json=sample_nexus_webhook)
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "ignored"
 
 
 @pytest.mark.asyncio