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>

config/locales/en.yml

@@ -372,6 +372,8 @@ en:
 
       Transcript:
       %{format_messages}
+  agent_capacity_policy:
+    inbox_already_assigned: 'Inbox has already been assigned to this policy'
   portals:
     send_instructions:
       email_required: 'Email is required'