feat: add PSA connection API endpoints with RBAC and tests

Five endpoints under /integrations/psa/connections:
- GET (any auth user), POST/PUT/DELETE/test (owner+ only)
- Create tests CW connection before saving; update re-tests on cred change
- Credentials decrypted only for masked hints in responses
- Three CI-safe tests: empty GET, engineer 403, delete 404

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

backend/app/api/router.py

@@ -17,6 +17,7 @@ from app.api.endpoints import ai_suggestions
 from app.api.endpoints import kb_accelerator
 from app.api.endpoints import beta_signup
 from app.api.endpoints import scripts
+from app.api.endpoints import integrations
 
 api_router = APIRouter()
 
@@ -58,3 +59,4 @@ api_router.include_router(ai_suggestions.router)
 api_router.include_router(kb_accelerator.router)
 api_router.include_router(beta_signup.router)
 api_router.include_router(scripts.router)
+api_router.include_router(integrations.router)