Write a front door for the repository

There was no README at the root, and the backend's own still pointed at an
onboarding guide that had been deleted. The new one says what the app is, how
to run it, and where the code lives, and states the test gates as they
actually are rather than as an older note claimed.

It records the two things that waste a newcomer's afternoon: the dev server
answers on localhost but not on the loopback address, because Vite binds IPv6
first, and the two design documents are largely superseded and say so inline.

backend/README.md

@@ -51,7 +51,7 @@ user and changes nothing; `--reset` only ever deletes the one demo workspace.
 
 `VacationDays` is implemented end-to-end (entity → schema → repo interface →
 SQLite impl → service → router → tests). **Copy this slice** when adding a new
-entity. See `JUNIOR_ONBOARDING.md` in the repo root for task-by-task guidance.
+entity.
 
 > The inline schema in `db/connection.py` exists only so dev/tests are
 > self-contained. Production schema is owned by Alembic migrations (a senior task).