chore(voice): drop non-UI changes from feat/whatsapp-call-ui

Reset BE/migration/spec files to base so this PR diffs as a pure UI change.
The deleted migration and the BE-side comment/edits belong on the BE PR
chain (feat/whatsapp-call-incoming-pipeline) rather than the UI PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/services/base/send_on_channel_service.rb

@@ -46,8 +46,7 @@ class Base::SendOnChannelService
   def invalid_message?
     # private notes aren't send to the channels
     # we should also avoid the case of message loops, when outgoing messages are created from channel
-    # voice_call bubbles are call status indicators, not deliverable messages — skip the send pipeline
-    # otherwise it falls through to "Template not found" / 24-hour-window errors
+    # voice_call bubbles are call status indicators, not deliverable messages
     message.private? || outgoing_message_originated_from_channel? || message.content_type == 'voice_call'
   end