feat(shared): auth_source constants and hq_user_id on User

Nothing writes them yet. They land now so phases 2 and 3 do not require a
second rebuild of every service that consumes the shared module.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/internal/models/user.go

@@ -10,4 +10,10 @@ const (
 	RoleMember = shared.RoleMember
 )
 
+const (
+	AuthLocal = shared.AuthLocal
+	AuthOIDC  = shared.AuthOIDC
+	AuthHQ    = shared.AuthHQ
+)
+
 func ValidRole(role string) bool { return shared.ValidRole(role) }