Fix the MQTT node, suggest message names, and drop Items

- MQTT nodes failed to build: the topic map was renamed to talk in ports, but
  __slots__ still declared the old name, so every MQTT node raised
  AttributeError. Building one of each node type is now a test, since __slots__
  makes this failure invisible until someone places the node.
- Port names offer the messages already in play: everything published is worth
  reading, and an input nobody provides yet is worth publishing. A message only
  connects when both ends spell it the same way, so choosing beats typing.
- Adding a port focuses its name field.
- Dragging onto an input that already reads something offers the extra port as
  well as the replacement — an MQTT or InfluxDB node usually wants both.
- The template's Item model, its routes, screens and table are gone.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016WzrvW7rjQbynnhF6pxh6i

backend/tests/conftest.py

@@ -7,7 +7,7 @@ from sqlmodel import Session, delete
 from app.core.config import settings
 from app.core.db import engine, init_db
 from app.main import app
-from app.models import Item, User
+from app.models import User
 from tests.utils.user import authentication_token_from_email
 from tests.utils.utils import get_superuser_token_headers
 
@@ -26,8 +26,6 @@ def db() -> Generator[Session, None, None]:
     with Session(engine) as session:
         init_db(session)
         yield session
-        statement = delete(Item)
-        session.execute(statement)
         statement = delete(User)
         session.execute(statement)
         session.commit()