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/fixtures/integration-fixtures/advisory/default.conf

@@ -0,0 +1,55 @@
+server {
+    listen 80;
+    server_name advisory-fixture.stella-ops.local;
+
+    default_type application/json;
+
+    # CERT-In (India) - unreachable from most networks
+    location /cert-in {
+        return 200 '{"status":"healthy","source":"cert-in","description":"CERT-In fixture proxy"}';
+    }
+
+    # FSTEC BDU (Russia) - unreachable from most networks
+    location /fstec-bdu {
+        return 200 '{"status":"healthy","source":"fstec-bdu","description":"FSTEC BDU fixture proxy"}';
+    }
+
+    # StellaOps Mirror - does not exist yet
+    location /stella-mirror {
+        return 200 '{"status":"healthy","source":"stella-mirror","version":"1.0.0","description":"StellaOps Advisory Mirror fixture"}';
+    }
+
+    # VEX Hub - local fixture
+    location /vex {
+        return 200 '{"status":"healthy","source":"vex","description":"VEX Hub fixture proxy"}';
+    }
+
+    # Exploit-DB - blocks automated requests
+    location /exploitdb {
+        return 200 '{"status":"healthy","source":"exploitdb","description":"Exploit-DB fixture proxy"}';
+    }
+
+    # AMD Security - blocks automated requests
+    location /amd {
+        return 200 '{"status":"healthy","source":"amd","description":"AMD Security fixture proxy"}';
+    }
+
+    # Siemens ProductCERT - blocks automated requests
+    location /siemens {
+        return 200 '{"status":"healthy","source":"siemens","description":"Siemens ProductCERT fixture proxy"}';
+    }
+
+    # Ruby Advisory DB (bundler-audit) - GitHub raw content issues
+    location /bundler-audit {
+        return 200 '{"status":"healthy","source":"bundler-audit","description":"Ruby Advisory DB fixture proxy"}';
+    }
+
+    # Catch-all health endpoint
+    location /health {
+        return 200 '{"status":"healthy","service":"advisory-fixture"}';
+    }
+
+    location / {
+        return 200 '{"status":"healthy","service":"advisory-fixture"}';
+    }
+}