fix: enforce forced retention on message edits, feedback, and error saves

Two more message-write paths bypassed ephemeral enforcement:

- The edit and feedback endpoints call updateMessage directly, without loading
  retention config, so editing an older permanent message after a switch to
  ephemeral left the message and its conversation non-temporary and visible.
  Load config on those routes and run a new applyForcedRetention helper after the
  update, which stamps the message and cascades the conversation/messages.

- The sendError and denyRequest middleware save messages with retention config
  but never call saveConvo, so a validation/model error or denied-request message
  could outlive its conversation. Pass capExpiryToConversation like the other
  message-only paths.

Extract the conversation cascade into a shared cascadeForcedConversationRetention
helper used by both saveMessage and applyForcedRetention.

api/server/middleware/error.js

@@ -56,6 +56,7 @@ const sendError = async (req, res, options, callback) => {
       { ...errorMessage, user },
       {
         context: 'api/server/utils/streamResponse.js - sendError',
+        capExpiryToConversation: true,
       },
     );
   }