docs(backend): document MongoDB connection and fail fast when it is missing

[peaceshallom37-rgb]

Closes #471

Summary

The backend connects to MongoDB via MONGODB_URI/MONGO_URI (ensureMongooseIndexes in backend/src/index.ts) and loads dozens of Mongoose models, but the connection string was undocumented — .env.example only listed PostgreSQL and Redis. A fresh deployment following the documented setup would never connect to MongoDB, and index sync + Mongoose-backed routes would silently behave as if no database existed.

Changes

• .env.example — adds a MONGODB_URI entry under Database Configuration, with a note explaining the Postgres-vs-Mongo split.
• backend/src/index.ts — ensureMongooseIndexes() now fails fast (throws before the server starts) when Mongoose models are registered but no MongoDB URI is configured, instead of silently returning. Connection handling when a URI is set is unchanged.
• backend/README.md — documents the division of responsibility: PostgreSQL is the primary relational store (users, enrollments, payments, courses, migrations); MongoDB is the document store for Mongoose-backed services (content delivery, notifications, offline sync, quizzes, translations, whiteboard, etc.), and is required at startup.

Verification

• npm run typecheck passes.
• Lint and test-suite failures are pre-existing on main (verified by stashing; e.g. @ts-ignore ban errors in index.ts and a featureFlags.ts import failure in tests/api.test.js).
• The fail-fast path is inside startServer(), which only runs when the module is the entrypoint (require.main === module), so tests that import the app are unaffected.