feat: implement full Prisma schema — users, workspaces, prompts, versions

[Ali7040]

Implement the full schema in apps/api/prisma/schema.prisma as defined in the repo.
Run and verify all migrations locally.

Models required for Phase 1:

• User, RefreshToken
• Workspace, WorkspaceMember, ApiKey
• Prompt, PromptVersion
• WorkspaceEvent (audit log)

Commands:

npx prisma migrate dev --name init
npx prisma db seed       # seed one test user + workspace
npx prisma studio        # verify schema visually

Acceptance criteria:

• All relations have correct cascade rules
• PromptVersion has contentHash unique per prompt (content-addressable)
• Seed script creates admin@promptgit.dev user with a demo workspace