fix: correct error handling, panic recovery, and JWT concurrency in proxy handler

[Copilot]

PR #1066 introduced several handler fixes but left auth errors mis-classified as 500s, swallowed http.ErrAbortHandler, returned plain-text panic responses, and introduced a data race on the JWT token cache.

Error handling: *req.ErrUnauthorized → 403 not 500

ResolveUserAndGroups returns *req.ErrUnauthorized for missing/invalid auth headers. Four call sites (authorizationMiddleware, registerModules handler, CheckUserInIgnoredGroupMiddleware, CheckUserInCapsuleGroupMiddleware) were routing all errors through HandleError (500). Now they mirror impersonateHandler:

var t *req.ErrUnauthorized
if errors.As(err, &t) {
    server.HandleUnauthorized(writer, err, msg)
} else {
    server.HandleError(writer, err, msg)
}

Panic recovery (recoveryMiddleware)

• http.ErrAbortHandler was swallowed (return); must be re-panicked so net/http can abort the connection correctly.
• Fallback 500 response was plain text via http.Error; now uses server.HandleError for a consistent Kubernetes-style JSON metav1.Status body.

JWT token cache: data race

invalidatedToken (sets.Set[string]) is captured by the middleware closure and mutated concurrently across requests. Fixed with a sync.RWMutex — reads under RLock, the invalidation write under Lock.

errors/panic.go

w.WriteHeader was hardcoding http.StatusForbidden / http.StatusInternalServerError rather than reading from the metav1.Status.Code field already set on the struct. Both handlers now use w.WriteHeader(int(status.Code)).