feat(voice): embed Call in voice_call message payload

Adds a nested `call` object to the voice_call message JSON / push
payload so the frontend can read call state directly from message.call
instead of duplicating it in content_attributes.data.

- Message gains has_one :call (inverse of existing Call#belongs_to :message)
  via enterprise module prepend.
- Message serializer (_message.json.jbuilder) emits a call block for
  voice_call messages when the Call is linked.
- Message#push_event_data mirrors the payload into socket broadcasts.
- MessageFinder eager-loads the call (+ contact + inbox.channel) to
  avoid N+1 when listing conversation messages.
- Call gains helper methods: display_status, from_number, to_number,
  recording_url, push_event_data. These centralize the direction- and
  status-aware derivations so jbuilder and push_event_data stay in sync.

Frontend continues to read content_attributes.data — this commit only
adds the new shape; a follow-up swaps readers to message.call.

app/models/message.rb

@@ -152,9 +152,14 @@ class Message < ApplicationRecord
     )
     data[:echo_id] = echo_id if echo_id.present?
     data[:attachments] = attachments.map(&:push_event_data) if attachments.present?
+    data[:call] = call.push_event_data if voice_call? && respond_to?(:call) && call.present?
     merge_sender_attributes(data)
   end
 
+  def voice_call?
+    content_type == 'voice_call'
+  end
+
   def conversation_push_event_data
     {
       assignee_id: conversation.assignee_id,