chore: migrate max_assignment_limit to agent capacity policies

Adds a data migration and rake task to convert inbox max_assignment_limit
settings into AgentCapacityPolicy records with per-inbox conversation
limits, preserving the exact capacity-based semantics.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

db/migrate/20260316000001_migrate_max_assignment_limit_to_policies.rb

@@ -0,0 +1,51 @@
+class MigrateMaxAssignmentLimitToPolicies < ActiveRecord::Migration[7.1]
+  def up
+    Account.find_each do |account|
+      migrate_account(account)
+    end
+  end
+
+  def down
+    AgentCapacityPolicy.where(name: 'Auto (Migrated)').destroy_all
+  end
+
+  private
+
+  def migrate_account(account)
+    inboxes_with_limit = account.inboxes
+                                .where("(auto_assignment_config->>'max_assignment_limit')::int > 0")
+
+    return if inboxes_with_limit.empty?
+
+    ActiveRecord::Base.transaction do
+      policy = AgentCapacityPolicy.find_or_create_by!(account: account, name: 'Auto (Migrated)') do |p|
+        p.description = 'Migrated from max_assignment_limit on inbox auto_assignment_config'
+      end
+
+      create_inbox_limits(policy, inboxes_with_limit)
+      assign_policy_to_inbox_members(account, policy, inboxes_with_limit)
+    end
+  end
+
+  def create_inbox_limits(policy, inboxes)
+    inboxes.each do |inbox|
+      next if InboxCapacityLimit.exists?(agent_capacity_policy_id: policy.id, inbox_id: inbox.id)
+
+      limit = inbox.auto_assignment_config['max_assignment_limit'].to_i
+
+      InboxCapacityLimit.create!(
+        agent_capacity_policy: policy,
+        inbox: inbox,
+        conversation_limit: limit
+      )
+    end
+  end
+
+  def assign_policy_to_inbox_members(account, policy, inboxes)
+    member_user_ids = InboxMember.where(inbox_id: inboxes.select(:id)).distinct.pluck(:user_id)
+
+    account.account_users
+           .where(user_id: member_user_ids, agent_capacity_policy_id: nil)
+           .find_each { |account_user| account_user.update!(agent_capacity_policy_id: policy.id) }
+  end
+end

lib/tasks/migrate_max_assignment_limit.rake

@@ -0,0 +1,11 @@
+# Manually run the max_assignment_limit to agent capacity policy migration.
+# This is idempotent and safe to run multiple times.
+#
+# Usage:
+#   bundle exec rake assignment:migrate_max_limit
+namespace :assignment do
+  desc 'Migrate max_assignment_limit from inbox config to agent capacity policies'
+  task migrate_max_limit: :environment do
+    MigrateMaxAssignmentLimitToPolicies.new.up
+  end
+end