chore: remove dead json imports (#27815)

Fourteen modules import `json` without using it. Ruff flags every one with F401, and a word-boundary search for `json` in each file matches only the import line itself, including inside strings, comments and annotations.

Two exclusions, both deliberate. Migration files are left alone: the import is equally dead there, but those files are frozen history and not worth the churn. `models/chats.py` has the same dead import and is handled in its own change, so it is skipped here to avoid two changes touching the same line.

No behaviour change.

backend/open_webui/models/channels.py

@@ -1,4 +1,3 @@
-import json
 import secrets
 import time
 import uuid

backend/open_webui/models/chat_messages.py

@@ -1,4 +1,3 @@
-import json
 import time
 import uuid
 from collections import Counter

backend/open_webui/models/groups.py

@@ -1,4 +1,3 @@
-import json
 import logging
 import time
 import uuid

backend/open_webui/models/knowledge.py

@@ -1,4 +1,3 @@
-import json
 import logging
 import time
 import uuid

backend/open_webui/models/messages.py

@@ -1,4 +1,3 @@
-import json
 import time
 import uuid
 from typing import Optional

backend/open_webui/models/notes.py

@@ -1,4 +1,3 @@
-import json
 import time
 import uuid
 from functools import lru_cache

backend/open_webui/models/prompt_history.py

@@ -1,7 +1,6 @@
 """Prompt history model for version tracking."""
 
 import difflib
-import json
 import time
 import uuid
 from typing import Optional