A new `auth_request` responder on the log store's host. nginx sends it
every `/select/logsql/` request whose `Authorization` is
`Bearer swarm-agent.<agent>.<secret>`; it answers 200 when the secret is
the one stored at `swarm/agents/<agent>/queue` (`swarm-agent-verify`),
401 otherwise. Every other bearer still goes to authelia, so agents
still presenting the hive's shared OIDC client keep working.
`auth_request` takes a fixed URI, so the split is a `map` on
`$http_authorization` plus an internal location that rewrites to the
map's pick. Checked against nginx 1.30.5 with stub upstreams: an agent
token only the stub validator accepts gets 200, a token only the stub
authelia accepts gets 200, every wrong one 401, and the internal
locations 404 from outside.
The responder serves a unix socket in its own traverse-only runtime
directory and reads the store under its own leaf
(`swarm-logs-auth`), whose policy is `swarm/agents/+/queue`, read-only.
glue-bao-tls mints the leaf where the store runs; elsewhere the
operator places it, and eval refuses a log store host without one.
Part of #4459.
1f4736e79b