Add usersFile (newUsersFile/load) for the JSON users-file shape
{"users":[{name,group,apiKey,limits?}]}, and authStore.maybeReload
for throttled hot reload: checks mtime at most every reloadEvery
(5s), reloads on change, keeps the last good set and logs via
errorf on any error (stat, parse, or a rejected replace), and logs
via logf when the reload changes the file-sourced user count.
nowFn is injectable for tests.
Wire-up in llmgateway.go: newGateway does a synchronous initial
load when Users.File is set (a bad initial file fails the
constructor); ServeHTTP calls g.auth.maybeReload() at entry
(no-op when no file is configured).
Also fixes replaceFileUsers's merge semantics: a file user now
overrides an inline user of the same name, even when their keys
differ, dropping the inline entry from the rebuilt set. The prior
digest-only keying let a same-named inline/file pair with
different keys coexist instead of the file version winning.
Add buildMu to serialize replaceFileUsers's writer body (including
buildEntry's potentially slow file-backed resolveSecret reads for
"file:"-prefixed API keys). Scope mu.Lock() to the final byDigest
swap only, so a rebuild in progress no longer stalls every identify
call for its whole duration. Behavior-preserving: writers stay fully
serialized, same as before.
Adds a regression test that holds buildMu externally and asserts
identify still succeeds — proving identify's read path does not
depend on the writer-serialization lock.
lukaszraczylo