feat: Add agent capacity controllers (#12200)

## Linear reference:
https://linear.app/chatwoot/issue/CW-4649/re-imagine-assignments

## Description
This PR introduces the foundation for Assignment V2 system by
implementing agent_capacity and their association with inboxes and
users.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

Test Coverage:
-  Controller specs for assignment policies CRUD operations
-  Enterprise-specific specs for balanced assignment order
-  Model specs for community/enterprise separation

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

enterprise/app/controllers/api/v1/accounts/agent_capacity_policies/inbox_limits_controller.rb

@@ -0,0 +1,47 @@
+class Api::V1::Accounts::AgentCapacityPolicies::InboxLimitsController < Api::V1::Accounts::EnterpriseAccountsController
+  before_action -> { check_authorization(AgentCapacityPolicy) }
+  before_action :fetch_policy
+  before_action :fetch_inbox, only: [:create]
+  before_action :fetch_inbox_limit, only: [:update, :destroy]
+  before_action :validate_no_duplicate, only: [:create]
+
+  def create
+    @inbox_limit = @agent_capacity_policy.inbox_capacity_limits.create!(
+      inbox: @inbox,
+      conversation_limit: permitted_params[:conversation_limit]
+    )
+  end
+
+  def update
+    @inbox_limit.update!(conversation_limit: permitted_params[:conversation_limit])
+  end
+
+  def destroy
+    @inbox_limit.destroy!
+    head :no_content
+  end
+
+  private
+
+  def fetch_policy
+    @agent_capacity_policy = Current.account.agent_capacity_policies.find(params[:agent_capacity_policy_id])
+  end
+
+  def fetch_inbox
+    @inbox = Current.account.inboxes.find(permitted_params[:inbox_id])
+  end
+
+  def fetch_inbox_limit
+    @inbox_limit = @agent_capacity_policy.inbox_capacity_limits.find(params[:id])
+  end
+
+  def validate_no_duplicate
+    return unless @agent_capacity_policy.inbox_capacity_limits.exists?(inbox: @inbox)
+
+    render_could_not_create_error(I18n.t('agent_capacity_policy.inbox_already_assigned'))
+  end
+
+  def permitted_params
+    params.permit(:inbox_id, :conversation_limit)
+  end
+end

enterprise/app/controllers/api/v1/accounts/agent_capacity_policies/users_controller.rb

@@ -0,0 +1,35 @@
+class Api::V1::Accounts::AgentCapacityPolicies::UsersController < Api::V1::Accounts::EnterpriseAccountsController
+  before_action -> { check_authorization(AgentCapacityPolicy) }
+  before_action :fetch_policy
+  before_action :fetch_user, only: [:destroy]
+
+  def index
+    @users = Current.account.users.joins(:account_users)
+                    .where(account_users: { agent_capacity_policy_id: @agent_capacity_policy.id })
+  end
+
+  def create
+    @account_user = Current.account.account_users.find_by!(user_id: permitted_params[:user_id])
+    @account_user.update!(agent_capacity_policy: @agent_capacity_policy)
+    @user = @account_user.user
+  end
+
+  def destroy
+    @account_user.update!(agent_capacity_policy: nil)
+    head :ok
+  end
+
+  private
+
+  def fetch_policy
+    @agent_capacity_policy = Current.account.agent_capacity_policies.find(params[:agent_capacity_policy_id])
+  end
+
+  def fetch_user
+    @account_user = Current.account.account_users.find_by!(user_id: params[:id])
+  end
+
+  def permitted_params
+    params.permit(:user_id)
+  end
+end

enterprise/app/controllers/api/v1/accounts/agent_capacity_policies_controller.rb

@@ -0,0 +1,37 @@
+class Api::V1::Accounts::AgentCapacityPoliciesController < Api::V1::Accounts::EnterpriseAccountsController
+  before_action :check_authorization
+  before_action :fetch_policy, only: [:show, :update, :destroy]
+
+  def index
+    @agent_capacity_policies = Current.account.agent_capacity_policies
+  end
+
+  def show; end
+
+  def create
+    @agent_capacity_policy = Current.account.agent_capacity_policies.create!(permitted_params)
+  end
+
+  def update
+    @agent_capacity_policy.update!(permitted_params)
+  end
+
+  def destroy
+    @agent_capacity_policy.destroy!
+    head :ok
+  end
+
+  private
+
+  def permitted_params
+    params.require(:agent_capacity_policy).permit(
+      :name,
+      :description,
+      exclusion_rules: [:overall_capacity, { hours: [], days: [] }]
+    )
+  end
+
+  def fetch_policy
+    @agent_capacity_policy = Current.account.agent_capacity_policies.find(params[:id])
+  end
+end