[server] Document the internal.admin flag

Complements https://github.com/ente-io/ente/pull/4228

server/configurations/local.yaml

@@ -317,6 +317,10 @@ internal:
     # List of user IDs that can use the admin API endpoints.
     # If this is not set, as a fallback, the first user is considered an admin.
     admins: []
+    # In case there is a single admin, it can be alternatively specified by as
+    # the "admin" instead of "admins". This can be useful e.g. when wishing to
+    # pass the admin as an environment variable.
+    admin:
 
 # Replication config
 #

server/pkg/middleware/auth.go

@@ -91,14 +91,16 @@ func (m *AuthMiddleware) AdminAuthMiddleware() gin.HandlerFunc {
 				return
 			}
 		}
-		// if no admins are set, then check if the user is first user in the system
+		// The config allows alternatively specifying a singular admin ID to
+		// workaround Viper issues in passing env vars for an int slice.
 		if len(admins) == 0 {
-			// Fallback for specifying a singular admin ID when encountering https://github.com/spf13/viper/issues/1611
 			if viper.GetInt("internal.admin") == userId {
 				c.Next()
 				return
 			}
-
+		}
+		// if no admins are set, then check if the user is first user in the system
+		if len(admins) == 0 {
 			id, err := m.UserAuthRepo.GetMinUserID()
 			if err != nil && id == userID {
 				c.Next()