Handle N+1 queries in SyncPolicyEventService

The following discussion from !172447 (merged) should be addressed:

• @jessieay started a discussion: (+4 comments)

  Question: is this an N+1? I believe it is.

  I asked Claude for a recommendation for something more computationally efficient and this is what was recommended:

  def affected_rules(protected_branch_id)
    # Early return if no branch ID specified - no computation needed
    rules = security_policy.approval_policy_rules.undeleted
    return rules if protected_branch_id.nil?
  
    # Pre-fetch all branch IDs to avoid repeated service calls
    rule_to_branches = rules.each_with_object({}) do |rule, mapping|
      mapping[rule] = sync_project_approval_policy_rules_service.protected_branch_ids(rule)
    end
  
    # Filter rules using the cached branch IDs
    rules.select do |rule|
      rule_to_branches[rule].include?(protected_branch_id)
    end
  end

• !172447 (comment 2284046031)

  Suggestion (non-blocking): now that this method is public, I wonder if it would be best moved somewhere outside of this service?