Root-cause fix for cross-file DB pollution in the vitest suite, which
surfaced as intermittent FK-violation failures once the suite hit 12
test files. tests/setup.ts previously mutated the global
process.env.DATABASE_URL and relied on src/lib/prisma.ts's singleton
picking it up, which is racy across files sharing a worker process
(process.env isn't reset between files even though Vitest resets each
file's module registry). fileParallelism: false only masked this.
Now each test file's own tests/setup.ts creates its own throwaway
Postgres database and vi.mock()s "@/lib/prisma" with a dedicated
PrismaClient constructed via the explicit datasourceUrl option, so no
mutable state is shared between files and fileParallelism can go back
to its default (true).
Verified with 4 consecutive full `npm test` runs (327/327 passing each
time) and no leftover throwaway databases after each run.
toanhoang
7e1351bde2