fix(api): downgrade Prisma to v6 — v7 requires driver adapter or Accelerate (#7)

Prisma 7 introduced a fundamental change: PrismaClient now requires either an
'adapter' (driver adapter using pg/postgres-js) or 'accelerateUrl' (Prisma's
paid Accelerate service). Plain DATABASE_URL in schema.prisma is also gone —
v7 wants it via prisma.config.ts.

The runtime crashed with:
  PrismaClientConstructorValidationError: Using engine type "client" requires
  either "adapter" or "accelerateUrl" to be provided to PrismaClient constructor.

Going back to Prisma 6, which still supports the simple 'url = env(...)' style
and a no-options PrismaClient — matching how the codebase is written. Adopting
v7's adapter-based model is a larger change that should land separately.

- apps/api/package.json: prisma + @prisma/client → ^6.19.3
- scripts/remote-deploy.sh: drop the hard-coded 'npx prisma@7.8.0 generate'
  pin so it uses whatever's in package.json

Co-authored-by: edneam <edneam>

Author: Dawn-Fighter