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/utils/item.py

@@ -1,16 +0,0 @@
-from sqlmodel import Session
-
-from app import crud
-from app.models import Item, ItemCreate
-from tests.utils.user import create_random_user
-from tests.utils.utils import random_lower_string
-
-
-def create_random_item(db: Session) -> Item:
-    user = create_random_user(db)
-    owner_id = user.id
-    assert owner_id is not None
-    title = random_lower_string()
-    description = random_lower_string()
-    item_in = ItemCreate(title=title, description=description)
-    return crud.create_item(session=db, item_in=item_in, owner_id=owner_id)