V-L1-I1: prune unimplemented backends (MySQL/Redis) from README, or stub them

## Context
README and DatabaseBackend enum disagree: README claims MySQL support; enum has Postgres/SQLite/MongoDB only.

## What to do
Either add MySQL to the enum + add a backend stub, or drop it from README. Pick one. Same for Redis (claimed in some places).

## Acceptance
- [ ] README only lists implemented (or stubbed) backends
