feat: Add unified Call model for voice calling (#14026)

Adds a Call model to track voice call state across providers (Twilio,
WhatsApp). This replaces storing call data in
conversation.additional_attributes and provides a foundation for call
analytics multi-call-per-conversation support, and future voice
providers.

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

db/migrate/20260408170902_create_calls.rb

@@ -0,0 +1,34 @@
+class CreateCalls < ActiveRecord::Migration[7.0]
+  def change
+    create_table :calls do |t|
+      t.bigint :account_id, null: false
+      t.bigint :inbox_id, null: false
+      t.bigint :conversation_id, null: false
+      t.bigint :contact_id, null: false
+      t.bigint :message_id
+      t.bigint :accepted_by_agent_id
+      t.string :provider_call_id, null: false
+      t.integer :provider, null: false, default: 0
+      t.integer :direction, null: false
+      t.string :status, null: false, default: 'ringing'
+      t.datetime :started_at
+      t.integer :duration_seconds
+      t.string :end_reason
+      t.jsonb :meta, default: {}
+      t.text :transcript
+
+      t.timestamps
+    end
+
+    add_call_indexes
+  end
+
+  private
+
+  def add_call_indexes
+    add_index :calls, [:provider, :provider_call_id], unique: true
+    add_index :calls, [:account_id, :conversation_id]
+    add_index :calls, [:account_id, :contact_id]
+    add_index :calls, :message_id
+  end
+end