doltsrht: serve /query and the api-meta.json beside it

The schema is mounted where /mcp is and for the same three reasons — before
web's same-origin group, before the cookie middleware, in a group of its own
— plus one of its own: it answers anonymous callers, so it cannot ride
core-go's WithSchema, whose router 401s an un-cookied request. It does need
the config and database middleware, which is why the group sits below them.

The file beside it is not optional. meta.sr.ht fetches api-meta.json from
every service it discovers and iterates each one's scopes to build the
personal-token page, so a service that mounts its own /query owes the
instance this too — and the scope list is what makes "dolt.sr.ht/repos:RO"
offerable there at all. The never-null rule that makes a mistake here a 500
for the WHOLE instance lives in ecore's apimeta; a test asserts the name this
service advertises is the one authn.RepoScope enforces.

README says what the surface serves and what it deliberately does not, and
that `hut graphql dolt` needs the patched hut — upstream segfaults on any
service outside its hard-coded list.