Adds Candidate (per-job download candidates found by the pipeline)
and LibraryItem (per-request imported album, deduped on
artist+album) tables. LibraryItem.requestId is @unique since
Request.libraryItem is a one-to-one back-relation (required for
Prisma validation, matching the existing Job.requestId pattern).
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