fix: package Prisma config for Railway deploys

[gabedalmolin]

Summary

Package the Prisma runtime configuration needed by Railway deployments so the pre-deploy migration step can resolve its datasource correctly in production containers.

What changed

• Copied prisma.config.ts and tsconfig.json into the runtime image used by Railway deployments.
• Updated prisma.config.ts to use the explicit Prisma 7 config shape with schema, migrations, and datasource.url.
• Added a build-time placeholder DATABASE_URL so prisma generate can run during image builds without depending on production secrets.
• Clarified in the Railway deployment guide that DATABASE_URL and REDIS_URL must be defined on the auth-api service itself.

Validation

• npm run typecheck
• docker build -t auth-api:prisma-runtime-check .
• npm run lint
• npm test
• npm run test:integration when the change affects infrastructure-backed paths

Risks/Notes

• This change is limited to deployment packaging and Prisma configuration; it does not change API behaviour.
• Railway still needs a real DATABASE_URL configured on the auth-api service at runtime.