perf: merge 3 conversation COUNT queries in /meta into single query (#13536)

# Pull Request Template

## Description


- Merges 3 separate COUNT queries in
`ConversationFinder#set_count_for_all_conversations` into a single query
using PostgreSQL `COUNT(*) FILTER (WHERE ...)`
- Every call to `/meta` and conversation index fires 3 COUNT queries
against the conversations table — one for "mine", one for "unassigned",
one for "all". These share the same base query and scan the same rows,
but execute as 3 independent round-trips.

from pg_stats

| Query | Calls | Total DB Time | Avg Latency |
|-------|-------|--------------|-------------|
| COUNT unassigned | 4.1M | 141,081 sec | 34ms |
| COUNT all | 4.1M | 132,302 sec | 32ms |
| COUNT mine | 3.7M | 37,637 sec | 10ms |
| **Total** | **~12M** | **311,020 sec** | |

`/meta` alone runs at 2.3K RPM (NewRelic), triggered on every WebSocket
event (conversation created/updated/status changed/assignee changed).



Fixes
https://linear.app/chatwoot/issue/INF-43/make-single-query-to-count-the-assigned-unassigned-and-all-instead-of

## Type of change

- [x] Performance fix

## How Has This Been Tested?

- [x] Verify conversation list shows correct
mine/unassigned/assigned/all counts
- [x] Verify /meta endpoint returns correct counts
- [x] Verify counts update correctly when assigning/unassigning
conversations
- [x] Verify counts with team filter applied
- [x] Monitor DB query count reduction in NewRelic after deploy

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] 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
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Sony Mathew <ynos1234@gmail.com>

app/finders/conversation_finder.rb

@@ -40,7 +40,7 @@ class ConversationFinder
   def perform
     set_up
 
-    mine_count, unassigned_count, all_count, = set_count_for_all_conversations
+    mine_count, unassigned_count, all_count = set_count_for_all_conversations
     assigned_count = all_count - unassigned_count
 
     filter_by_assignee_type
@@ -184,6 +184,17 @@ class ConversationFinder
   end
 
   def set_count_for_all_conversations
+    return legacy_count_for_all_conversations if @conversations.limit_value || @conversations.offset_value || @conversations.eager_loading?
+
+    counts = @conversations.unscope(:order).pick(
+      Arel.sql("COUNT(*) FILTER (WHERE assignee_id = #{current_user.id})"),
+      Arel.sql('COUNT(*) FILTER (WHERE assignee_id IS NULL)'),
+      Arel.sql('COUNT(*)')
+    )
+    counts || [0, 0, 0]
+  end
+
+  def legacy_count_for_all_conversations
     [
       @conversations.assigned_to(current_user).count,
       @conversations.unassigned.count,