chore: remove unused telegram bot model (#12417)

## Summary
- remove unused TelegramBot model and its association
- drop obsolete telegram_bots table

db/migrate/20250826000000_drop_telegram_bots.rb

@@ -0,0 +1,10 @@
+class DropTelegramBots < ActiveRecord::Migration[7.1]
+  def change
+    drop_table :telegram_bots do |t|
+      t.string :name
+      t.string :auth_key
+      t.integer :account_id
+      t.timestamps
+    end
+  end
+end