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/models/agent_capacity_policy.rb

@@ -0,0 +1,27 @@
+# == Schema Information
+#
+# Table name: agent_capacity_policies
+#
+#  id              :bigint           not null, primary key
+#  description     :text
+#  exclusion_rules :jsonb            not null
+#  name            :string(255)      not null
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#  account_id      :bigint           not null
+#
+# Indexes
+#
+#  index_agent_capacity_policies_on_account_id  (account_id)
+#
+class AgentCapacityPolicy < ApplicationRecord
+  MAX_NAME_LENGTH = 255
+
+  belongs_to :account
+  has_many :inbox_capacity_limits, dependent: :destroy
+  has_many :inboxes, through: :inbox_capacity_limits
+  has_many :account_users, dependent: :nullify
+
+  validates :name, presence: true, length: { maximum: MAX_NAME_LENGTH }
+  validates :account, presence: true
+end

enterprise/app/models/enterprise/concerns/account.rb

@@ -5,6 +5,7 @@ module Enterprise::Concerns::Account
     has_many :sla_policies, dependent: :destroy_async
     has_many :applied_slas, dependent: :destroy_async
     has_many :custom_roles, dependent: :destroy_async
+    has_many :agent_capacity_policies, dependent: :destroy_async
 
     has_many :captain_assistants, dependent: :destroy_async, class_name: 'Captain::Assistant'
     has_many :captain_assistant_responses, dependent: :destroy_async, class_name: 'Captain::AssistantResponse'

enterprise/app/models/enterprise/concerns/account_user.rb

@@ -3,5 +3,6 @@ module Enterprise::Concerns::AccountUser
 
   included do
     belongs_to :custom_role, optional: true
+    belongs_to :agent_capacity_policy, optional: true
   end
 end

enterprise/app/models/inbox_capacity_limit.rb

@@ -0,0 +1,24 @@
+# == Schema Information
+#
+# Table name: inbox_capacity_limits
+#
+#  id                       :bigint           not null, primary key
+#  conversation_limit       :integer          not null
+#  created_at               :datetime         not null
+#  updated_at               :datetime         not null
+#  agent_capacity_policy_id :bigint           not null
+#  inbox_id                 :bigint           not null
+#
+# Indexes
+#
+#  idx_on_agent_capacity_policy_id_inbox_id_71c7ec4caf      (agent_capacity_policy_id,inbox_id) UNIQUE
+#  index_inbox_capacity_limits_on_agent_capacity_policy_id  (agent_capacity_policy_id)
+#  index_inbox_capacity_limits_on_inbox_id                  (inbox_id)
+#
+class InboxCapacityLimit < ApplicationRecord
+  belongs_to :agent_capacity_policy
+  belongs_to :inbox
+
+  validates :conversation_limit, presence: true, numericality: { greater_than: 0, only_integer: true }
+  validates :inbox_id, uniqueness: { scope: :agent_capacity_policy_id }
+end