feat: Assignment service (v2) (#12320)

## Linear Link

 
## Description

This PR introduces a new robust auto-assignment system for conversations
in Chatwoot. The system replaces the existing round-robin assignment
with a more sophisticated service-based architecture that supports
multiple assignment strategies, rate limiting, and Enterprise features
like capacity-based assignment and balanced distribution.

## Type of change

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

## How Has This Been Tested?

- Unit test cases
- Test conversations getting assigned on status change to open
- Test the job directly via rails console

## 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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds a new service-based auto-assignment system with scheduled jobs,
rate limiting, enterprise capacity/balanced selection, and wiring via
inbox/handler; includes Redis helpers and comprehensive tests.
> 
> - **Auto-assignment v2 (core services)**:
> - Add `AutoAssignment::AssignmentService` with bulk assignment,
configurable conversation priority, RR selection, and per-agent rate
limiting via `AutoAssignment::RateLimiter`.
>   - Add `AutoAssignment::RoundRobinSelector` for agent selection.
> - **Jobs & scheduling**:
> - Add `AutoAssignment::AssignmentJob` (per-inbox bulk assign;
env-based limit) and `AutoAssignment::PeriodicAssignmentJob` (batch over
accounts/inboxes).
> - Schedule periodic run in `config/schedule.yml`
(`periodic_assignment_job`).
> - **Model/concerns wiring**:
> - Include `InboxAgentAvailability` in `Inbox`; add
`Inbox#auto_assignment_v2_enabled?`.
> - Update `AutoAssignmentHandler` to trigger v2 job when
`auto_assignment_v2_enabled?`, else fallback to legacy.
> - **Enterprise extensions**:
> - Add `Enterprise::InboxAgentAvailability` (capacity-aware filtering)
and `Enterprise::Concerns::Inbox` association `inbox_capacity_limits`.
> - Extend service via `Enterprise::AutoAssignment::AssignmentService`
(policy-driven config, capacity filtering, exclusion rules) and add
selectors/services: `BalancedSelector`, `CapacityService`.
> - **Infrastructure**:
> - Enhance `Redis::Alfred` with `expire`, key scan/count, and extended
ZSET helpers (`zadd`, `zcount`, `zcard`, `zrangebyscore`).
> - **Tests**:
> - Add specs for jobs, core service, rate limiter, RR selector, and
enterprise features (capacity, balanced selection, exclusions).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0ebe187c8aea73765b0122a44b18d6f465c2477f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

enterprise/app/models/enterprise/concerns/inbox.rb

@@ -6,5 +6,6 @@ module Enterprise::Concerns::Inbox
     has_one :captain_assistant,
             through: :captain_inbox,
             class_name: 'Captain::Assistant'
+    has_many :inbox_capacity_limits, dependent: :destroy
   end
 end

enterprise/app/models/enterprise/inbox_agent_availability.rb

@@ -0,0 +1,31 @@
+module Enterprise::InboxAgentAvailability
+  extend ActiveSupport::Concern
+
+  def member_ids_with_assignment_capacity
+    return member_ids unless capacity_filtering_enabled?
+
+    # Get online agents with capacity
+    agents = available_agents
+    agents = filter_by_capacity(agents)
+    agents.map(&:user_id)
+  end
+
+  private
+
+  def filter_by_capacity(inbox_members_scope)
+    return inbox_members_scope unless capacity_filtering_enabled?
+
+    inbox_members_scope.select do |inbox_member|
+      capacity_service.agent_has_capacity?(inbox_member.user, self)
+    end
+  end
+
+  def capacity_filtering_enabled?
+    account.feature_enabled?('assignment_v2') &&
+      account.account_users.joins(:agent_capacity_policy).exists?
+  end
+
+  def capacity_service
+    @capacity_service ||= Enterprise::AutoAssignment::CapacityService.new
+  end
+end