Move backend tests from Jest to Vitest

The frontend already runs Vitest, the backend runs Jest with ts-jest. One runner for the whole monorepo is simpler, and Vitest is a much better fit once the backend is on Hono (native ESM, no ts-jest transform).

• Port jest.config.ts to vitest.config.ts, keep the path aliases and the DB test guard
• Replace supertest with Hono's app.request() where practical, no listening server needed
• Port the jest.mock calls to vi.mock
• Remove jest, ts-jest, @jest/globals, supertest and @types/supertest
• Drop the forceExit workaround, teardown should close the pool properly instead