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

[Fury03]

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.

[Fury03]

Note on the failing CI checks

The red checks here are pre-existing on main and unrelated to this PR.

main's own CI run — 30494417974, the merge commit for #1130, which is the base of this branch — fails with the identical set:

Job	Cause on main
Backend CI	~20 tsc errors, mostly IndexerStateRow.createdAt missing in src/lib/indexer-state.ts, plus soroban-indexer.service.ts, soroban-event-worker.ts, admin-rate-limiter.middleware.ts, tests/stream.test.ts, tests/stream.validator.test.ts
Frontend CI	eslint parsing errors in frontend/src/components/dashboard/dashboard-view.tsx, frontend/src/hooks/useIncomingStreams.test.ts, frontend/src/lib/dashboard.ts
Soroban Contracts CI	cargo fmt diffs at contracts/stream_contract/src/test.rs:433 and :2709
Backend Docker Image CI	fails during the backend build, same tsc errors
Backend npm test	14 failing tests across 6 files: indexer-state, soroban-event-worker, integration/admin-metrics, integration/stream-actions, integration/stream-lifecycle, integration/streams/withdraw

This PR changes only Markdown, so it cannot affect any of those jobs — a docs edit does not change cargo fmt output or TypeScript types.

Happy to open a separate PR fixing the main breakage if that would be useful, but it would touch files well outside this issue's scope, so I have kept it out of here.