fix: enforce message ownership in group/DM channel update + delete endpoints (#24506)

* fix: enforce message ownership in group/DM channel update + delete endpoints

`update_message_by_id` (channels.py:1348) and `delete_message_by_id`
(channels.py:1550) branch on `channel.type`. The `else` branch (standard
channels) correctly enforces `message.user_id != user.id` ownership before
mutating, but the `if channel.type in ['group', 'dm']` branch only checked
`is_user_channel_member` — channel membership alone, with no message
ownership verification.

Effect on group/DM channels: any verified member of the conversation could:

- overwrite another member's message content while the server preserved
  `user_id=victim`, producing tampered content that renders to other
  members as the original author's authentic post (integrity + authenticity);
- silently delete another member's messages, removing them from
  conversation history without trace (integrity).

Reproduced end-to-end against v0.9.4 with three users (attacker, victim,
viewer) sharing a group channel: attacker overwrites victim's message and
deletes another, viewer reads the tampered content as victim-authored.

Two patches, identical shape, mirror the `else` branch's existing
ownership semantics:

- `update_message_by_id` group/DM branch: add
  `if user.role != 'admin' and message.user_id != user.id: raise 403`
  immediately after the `is_user_channel_member` check.
- `delete_message_by_id` group/DM branch: same.

The standard-channel branch is unchanged (it already enforced ownership).
Admins remain able to moderate any message, matching the existing semantic
in the standard-channel branch.

Reports consolidated under GHSA-wwhq-cx22-f7vv (earliest live filing of the
group/DM-specific variant). Same gap previously surfaced and partially
fixed under GHSA-jxwr-g6r6-j3fx (which addressed the standard-channel
branch only) — this completes the cohort.

* chore: trim comments

backend/open_webui/routers/channels.py

@@ -1369,6 +1369,9 @@ async def update_message_by_id(
     if channel.type in ['group', 'dm']:
         if not await Channels.is_user_channel_member(channel.id, user.id, db=db):
             raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
+        # Membership is not authorship — block cross-member edits.
+        if user.role != 'admin' and message.user_id != user.id:
+            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
     else:
         if (
             user.role != 'admin'
@@ -1570,6 +1573,9 @@ async def delete_message_by_id(
     if channel.type in ['group', 'dm']:
         if not await Channels.is_user_channel_member(channel.id, user.id, db=db):
             raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
+        # Membership is not authorship — block cross-member deletes.
+        if user.role != 'admin' and message.user_id != user.id:
+            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
     else:
         if (
             user.role != 'admin'