fix(federation): Handle permission mismatches for reaction permissions

[luflow]

Summary

This PR fixes issues when federated instances are at different versions and have different permission bits (e.g., one instance has the new REACT permission from the migration, while the other doesn't yet).

Problem

When the reaction permission feature is deployed across federated Nextcloud instances at different version levels:

1. Your instance (v34) updates before host (v33): Local permissions include PERMISSIONS_REACT due to migration, but host doesn't understand it. The middleware permission check fails locally even though the host would accept the request.

2. Host updates after you: The host's migration adds REACT permission to federated attendees, but the remote participant's local copy doesn't get updated (migrations don't trigger federation notifications).

3. Host updates before you: Your migration might accidentally grant REACT permission that a moderator had intentionally restricted.

Solution

• Skip local permission checks for federated conversations: Since requests are proxied to the host server anyway, the host validates permissions authoritatively. This prevents false negatives from version mismatches.

• Add permission healing on federated room join: When a user joins a federated room, sync local permissions with the host's authoritative permissions. This ensures permissions stay in sync across version differences.

Test plan

• User on v34 instance can react in federated conversation hosted on v33 instance
• User on v33 instance can react in federated conversation hosted on v34 instance
• Permissions are synced when joining a federated room after either instance upgrades
• Moderator permission restrictions are respected after both instances upgrade

Fixes: #16902