Add integration connector plugins and compose fixtures

Scaffold connector plugins for DockerRegistry, GitLab, Gitea,
Jenkins, and Nexus. Wire plugin discovery in IntegrationService
and add compose fixtures for local integration testing.

- 5 new connector plugins under src/Integrations/__Plugins/
- docker-compose.integrations.yml for local fixture services
- Advisory source catalog and source management API updates
- Integration e2e test specs and Playwright config
- Integration hub docs under docs/integrations/

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

devops/compose/docker-compose.integration-fixtures.yml

@@ -57,3 +57,25 @@ services:
     labels:
       com.stellaops.profile: "qa-fixtures"
       com.stellaops.environment: "local-qa"
+
+  advisory-fixture:
+    image: nginx:1.27-alpine
+    container_name: stellaops-advisory-fixture
+    restart: unless-stopped
+    ports:
+      - "127.1.1.8:80:80"
+    volumes:
+      - ./fixtures/integration-fixtures/advisory/default.conf:/etc/nginx/conf.d/default.conf:ro
+    networks:
+      stellaops:
+        aliases:
+          - advisory-fixture.stella-ops.local
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://127.0.0.1/health | grep -q 'healthy'"]
+      interval: 15s
+      timeout: 5s
+      retries: 5
+      start_period: 5s
+    labels:
+      com.stellaops.profile: "qa-fixtures"
+      com.stellaops.environment: "local-qa"