docs(backend): document prisma.config.ts and its role alongside schema.prisma - #1146

Closes #1095

Problem

backend/prisma.config.ts configures the Prisma CLI (schema path, migrations path, datasource URL) separately from the data model in backend/prisma/schema.prisma, but neither backend/README.md nor docs/DEVELOPMENT.md mentioned the file. A contributor debugging a prisma generate/prisma migrate quirk had no pointer to check it.

Changes

backend/README.md — new subsection under Prisma Database:

• A table splitting responsibilities: schema.prisma owns the data model, prisma.config.ts owns CLI configuration.
• Why it's a separate file: it's TypeScript evaluated by Node before the CLI runs, so it can read environment variables. Prisma 7 no longer auto-loads .env for CLI commands, which is why the file opens with import "dotenv/config".
• What it currently sets (schema, migrations.path, datasource.url).
• A short troubleshooting list for the failures that actually trace back to this file: missing/wrong DATABASE_URL, paths resolving relative to backend/, and the seed command still living in the legacy prisma.seed field of package.json rather than migrations.seed in the config.

docs/DEVELOPMENT.md — two links into that section:

• In the backend database setup step, right after the prisma:generate / prisma:migrate commands.
• In the Database migration issues troubleshooting list.

Out of scope

No changes to the Prisma config itself, and no migration to a different config format — docs only.