test: integrate prisma-mock for unit tests

[Copilot]

🔗 Linked issue

Resolves setup-database-for-tests

📚 Description

Replaced vitest-mock-extended with prisma-mock for unit tests to eliminate PostgreSQL dependency and improve test execution speed.

Changes:

• Prisma schema: Added DMMF generator to export database metadata required by prisma-mock
• Unit tests: Migrated to prisma-mock with per-test fresh instances and in-test data seeding
  • server/journals/journal.service.spec.ts
  • server/documents/user.document.service.spec.ts
• Documentation: Added docs/testing-with-prisma-mock.md with implementation guide

Usage pattern:

import createPrismaMock from 'prisma-mock/client'
import { Prisma, type PrismaClient } from '../database'
import * as dmmf from '../database/generated/dmmf'

beforeEach(() => {
  const mockClient = mockDeep<PrismaClient>()
  prisma = createPrismaMock(Prisma, {
    mockClient,
    datamodel: { enums: dmmf.enums, models: dmmf.models }
  })
  // Seed test data
  await prisma.journal.create({ data: testJournal })
})

Integration tests remain on PostgreSQL - they exercise complex nested queries and full-stack behavior that prisma-mock doesn't fully support.

Original prompt

Use https://github.com/demonsters/prisma-mock for setting up the database for unit and integration tests.

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[tobiasdiez]

#2486 makes more sense