Adds the Prisma schema (Request/Job models, RequestStatus/JobState
enums), a Prisma client singleton at web/src/lib/db.ts, and the
init_request_job migration, applied and verified against Postgres.
Also publishes the db service's port (5432:5432) in docker-compose.yml
so host-run Prisma commands can reach localhost:5432 for local dev.
Jonathan