test(hub): isolate Vitest state and worker databases #111

Hub contributors need clean, independent test cases while files run in parallel. Centralize reset of reusable test state before each case and remove redundant local cleanup hooks, retaining explicit disposal where tests own real resources.

Add a dedicated database-enabled server Vitest project. Only that project requires Postgres. Prepare the current Prisma schema before database tests run, provide isolated databases for each Vitest worker, and clear database records before every case. Tests arrange their own records, act, and assert. Protect developer databases from destructive cleanup and avoid collisions between simultaneous test runs. Document the test lifecycle and local/CI commands.

Implement as a separate PR stacked on #108. Browser and pure unit tests remain database-free.