[ENH] Postgres Stack

[b-jonathan]

Is your feature request related to a problem?
I want to be able to use Postgres

Describe the solution

Add PostgreSQL as a new database option alongside MongoDB.

Requirements:

• Connection setup: Prompt user to choose between local (default localhost:5432) or custom connection
  string (same pattern as MongoDB's local vs auth URI flow)
• Use Prisma as the ORM — run prisma init, generate a starter schema with a sample User model, and
  generate the Prisma client
• Compatible with Express backend only (add templates under postgresql/express/)
• ValidateDependency should gate incompatible backend combinations
• Generate a seed route similar to MongoDB's (query the sample model)
• Append DATABASE_URL to the backend .env file via AppendUniqueLines
• Add prisma/ and .env to .gitignore
• Register as "postgres" in the stack registry