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>

spec/factories/agent_capacity_policies.rb

@@ -0,0 +1,21 @@
+FactoryBot.define do
+  factory :agent_capacity_policy do
+    account
+    sequence(:name) { |n| "Agent Capacity Policy #{n}" }
+    description { 'Test agent capacity policy' }
+    exclusion_rules { {} }
+
+    trait :with_overall_capacity do
+      exclusion_rules { { 'overall_capacity' => 10 } }
+    end
+
+    trait :with_time_exclusions do
+      exclusion_rules do
+        {
+          'hours' => [0, 1, 2, 3, 4, 5],
+          'days' => %w[saturday sunday]
+        }
+      end
+    end
+  end
+end

spec/factories/inbox_capacity_limits.rb

@@ -0,0 +1,7 @@
+FactoryBot.define do
+  factory :inbox_capacity_limit do
+    association :agent_capacity_policy, factory: :agent_capacity_policy
+    inbox
+    conversation_limit { 5 }
+  end
+end