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/routers/channels.py

@@ -1,6 +1,5 @@
 import base64
 import io
-import json
 import logging
 from typing import Optional