Add channel-scoped conversations for iMessage, WhatsApp, and email

Revert get_conversation_for_user to use Conversation.get() with
MultipleObjectsReturned fallback. Add channel field to Conversation
model and get_conversation_for_channel helper so each messaging
channel gets its own isolated conversation per user.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blueprints/conversation/models.py

@@ -21,6 +21,7 @@ class Conversation(Model):
     user: fields.ForeignKeyRelation = fields.ForeignKeyField(
         "models.User", related_name="conversations", null=True
     )
+    channel = fields.CharField(max_length=20, default="web", null=True)
 
     class Meta:
         table = "conversations"