web: rename a database from its settings page

A database's name lives in two places — the metadata row and the on-disk
store directory its path names — so a rename has to move both. The order
is creation's: the row first, where a name already taken is refused by the
unique index before anything on disk changes, then the store, then the
handle the remotesapi memoized under the old path. A store that will not
move rolls the row back, so the two halves never disagree about where a
database lives; only a failed rollback is escalated to a human, because
it is the one outcome no later request can repair.

storage.MoveStore is one os.Rename behind the containment guard DeleteStore
already used, extracted here as containedPath. It refuses an occupied
destination outright: os.Rename over an empty directory would succeed and
swallow it.

No redirect is left behind — the old address simply stops resolving, as on
git.sr.ht — so an existing clone needs its remote replaced, and a companion
of a git repository is re-provisioned under its old name by the next push
to that repository. The README's quickstart now says both.