fix(messages): add message to existing 1:1 conversation from compose modal

When composing a new message to someone you already have a conversation
with, the dedup logic returned the existing conversation without adding
the message. Now it creates the message and publishes SSE notification.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

blueprints/messages/conversation_routes.py

@@ -271,7 +271,33 @@ def api_conversations_create():
             for conv in existing:
                 conv_member_ids = {m.user_id for m in conv.members}
                 if conv_member_ids == {current_user.id, other_id}:
-                    # Return existing conversation
+                    # Existing 1:1 — add message if provided
+                    if message_text:
+                        now = datetime.now()
+                        msg = ConvMessage(
+                            conversation_id=conv.id,
+                            sender_id=current_user.id,
+                            content=message_text,
+                            created_at=now,
+                        )
+                        db.add(msg)
+                        db.flush()
+                        conv.last_message_id = msg.id
+                        conv.updated_at = now
+                        db.commit()
+                        db.refresh(conv)
+
+                        sender_name = current_user.name or current_user.email.split('@')[0]
+                        _publish_to_conv(db, conv.id, 'new_message', {
+                            'conversation_id': conv.id,
+                            'message': {
+                                'id': msg.id,
+                                'content_preview': strip_html(message_text)[:100],
+                                'sender_name': sender_name,
+                                'sender_id': current_user.id,
+                            },
+                        }, exclude_user_id=current_user.id)
+
                     membership = db.query(ConversationMember).filter_by(
                         conversation_id=conv.id,
                         user_id=current_user.id,