Background User merging

[cubap]

Copilot is concerned we'll get too many users at once and block ourselves.

The authentication middleware now handles temporary user merging, which could be a slow operation involving multiple database queries and group updates. This blocking operation happens on every authenticated request for new users. Consider offloading this to a background job or showing a loading state to the user, as this could cause request timeouts for users with many project invitations.

          // Offload the merge and save to a background job to avoid blocking the request
          user.data.mergePending = true;
          req.user = user;
          // Start the merge in the background
          setImmediate(async () => {
            try {
              await user.mergeFromTemporaryUser(existingUser);
              await user.save();
              // Optionally, clear the mergePending flag here if you want to update the user record
            } catch (err) {
              console.error("Background mergeFromTemporaryUser failed:", err);
            }
          });
          next();

Originally posted by @Copilot in #431