perf: Add database index on conversations identifier (#12715)

**Problem**
Slack webhook processing was failing with 500 errors due to database
timeouts. The query `Conversation.where(identifier:
params[:event][:thread_ts]).first` was performing full table scans and
hitting PostgreSQL statement timeout.

**Solution**
Added database index on conversations.identifier and account_id.

db/migrate/20251022152158_add_index_to_conversations_identifier.rb

@@ -0,0 +1,6 @@
+class AddIndexToConversationsIdentifier < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+  def change
+    add_index :conversations, [:identifier, :account_id], name: 'index_conversations_on_identifier_and_account_id', algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_10_03_091242) do
+ActiveRecord::Schema[7.1].define(version: 2025_10_22_152158) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -676,6 +676,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_10_03_091242) do
     t.index ["contact_id"], name: "index_conversations_on_contact_id"
     t.index ["contact_inbox_id"], name: "index_conversations_on_contact_inbox_id"
     t.index ["first_reply_created_at"], name: "index_conversations_on_first_reply_created_at"
+    t.index ["identifier", "account_id"], name: "index_conversations_on_identifier_and_account_id" 
     t.index ["inbox_id"], name: "index_conversations_on_inbox_id"
     t.index ["priority"], name: "index_conversations_on_priority"
     t.index ["status", "account_id"], name: "index_conversations_on_status_and_account_id"