Add @-mention highlighting, sidebar badge, and push customization (#39)

@username tokens in a sent message are parsed against the room's actual
members (skipping fenced/inline code, so pasted code isn't misread) and
recorded as MessageMention rows, reusing #38's read-tracking and
offline-member broadcast infrastructure rather than building a parallel
notification path:

- Sidebar: a mentioned-and-unread room shows a distinct highlight-
  colored badge instead of (not alongside) the plain unread dot --
  computed the same way as has_unread, just scoped to messages that
  mention the caller, and cleared by the same last_read_at mark-read
  flow.
- Push notifications: a mentioned offline recipient gets "X mentioned
  you: ..." instead of the generic "X: ...", still per-recipient since
  the same message can page some room members and not others.
- Message rendering: a validated @username is highlighted inline,
  implemented by turning it into a `[@username](mention:username)` link
  before markdown parsing and overriding link rendering to style
  `mention:`-scheme links as a span instead of an anchor -- reuses
  markdown-to-jsx's existing parser rather than hand-rolling text-node
  splitting.
- Composer: typing @ opens an autocomplete dropdown of matching room
  members (arrow keys to navigate, Enter/Tab/click to insert, Escape or
  moving the cursor away to dismiss).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/message_service.py

@@ -6,8 +6,9 @@ from sqlalchemy import or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from app.models import Message, MessageReaction
+from app.models import Message, MessageMention, MessageReaction
 from app.schemas.message import ReactionSummary
+from app.services.mention_service import extract_mentioned_user_ids
 
 
 class MessageNotFoundError(Exception):
@@ -30,6 +31,12 @@ async def create_message(
         room_id=room_id, user_id=user_id, content=content, image_id=image_id, file_id=file_id
     )
     db.add(message)
+    # message.id is available immediately (a Python-side uuid4 default, not
+    # server-generated), so mention rows can reference it without a flush.
+    if content:
+        mentioned_ids = await extract_mentioned_user_ids(db, room_id, content)
+        for mentioned_id in mentioned_ids:
+            db.add(MessageMention(message_id=message.id, user_id=mentioned_id))
     await db.commit()
     await db.refresh(message)
     return message