fix(backend): Eliminate N+1 queries for reaction_added events

[sentry]

This PR addresses an N+1 query issue occurring during the processing of reaction_added events, specifically when saving messages.

Problem:
When a reaction_added event with multiple receivers was processed, the AwardService::gibToUser() method would save messages in a loop. Each saveOrFail() call triggered CakePHP's ORM validation rules (existsIn for sender_user_id and receiver_user_id) defined in MessagesTable::buildRules(). This resulted in two SELECT 1 queries to check for user existence for each message being saved, leading to an N+1 query pattern (where N is the number of receivers).

Solution:
Since the fromUser and toUser entities are already loaded and their existence is guaranteed before gibToUser() is called, the ORM's existsIn validation checks are redundant. The fix involves:

1. Passing ['validate' => false] to the saveOrFail() method in AwardService::gibToUser(). This bypasses the unnecessary ORM-level existence checks.
2. Adding a comment to MessagesTable::buildRules() to clarify that while the existsIn rules are defined, they can be conditionally skipped when the caller guarantees entity existence, relying on database-level foreign key constraints for referential integrity.

This change significantly reduces the number of database queries for reaction_added events with multiple receivers, improving performance.