test: cover graph investigation real stack

[udgover]

Summary

• add a disposable Playwright path for the dedicated graph workspace
• create synthetic objects and directed relationships through the real API
• launch from Explore in graph, verify exact evidence, add a second anchor, and assert no graph mutation call
• delete synthetic objects and rely on the existing safe harness for complete stack cleanup

Dependencies

Stacked on #36 and requires yeti-platform/yeti#1338 plus yeti-platform/yeti-feeds-frontend#303. Safe merge order is backend, frontend, then this coverage PR.

Validation

• TypeScript/Playwright discovery and Compose rendering passed
• INTEGRATION_FRONTEND_PORT=18081 make test-integration: 10 passed
• teardown removed the disposable containers, network, and volumes

Security and data handling

The scenario uses generated synthetic values only, obtains credentials through the existing test harness, checks that investigation sends no POST, PATCH, PUT, or DELETE graph mutation request, and cleans the created objects in a finally block.

Rollback

Revert this test-only commit. Production Compose behavior and persistence are unchanged.