Add #roomname references in chat messages (#47)

Mirrors the existing @-mention system's shape: a regex finds #roomname
tokens, extract_referenced_room_ids validates them against rooms the
*sender* actually belongs to (mirrors mentions' "must be a real member"
rule -- referencing a private room the sender isn't in would otherwise
leak its existence), and a MessageRoomReference join row is stored per
match in create_message. No notification/unread layer, unlike mentions --
referencing a room has no "you were referenced" semantics.

Rendering is the same markdown-link rewrite trick MessageContent.tsx
already uses for mentions (#username -> [#username](mention:username)),
but resolved against the *viewer's* own room list (threaded down from
ChatShellPage's room state through ChatPane/MessageList) rather than the
stored server-side reference -- a reference to a room the current viewer
isn't in quietly renders as plain text instead of a link, same as an
@mention of someone outside the room does. The href scheme renders a
real react-router Link instead of mentions' inert span, since a room
reference is meant to be navigable.

mention_service.strip_code_spans (was _strip_code_spans) is now shared
between both extraction paths rather than private to one module.

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

backend/app/services/message_service.py

@@ -6,10 +6,11 @@ from sqlalchemy import or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
-from app.models import Message, MessageMention, MessageReaction
+from app.models import Message, MessageMention, MessageReaction, MessageRoomReference
 from app.schemas.message import ReactionSummary
 from app.services.link_preview_service import extract_first_url
 from app.services.mention_service import extract_mentioned_user_ids
+from app.services.room_reference_service import extract_referenced_room_ids
 
 
 class MessageNotFoundError(Exception):
@@ -43,6 +44,9 @@ async def create_message(
         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))
+        referenced_room_ids = await extract_referenced_room_ids(db, user_id, content)
+        for referenced_room_id in referenced_room_ids:
+            db.add(MessageRoomReference(message_id=message.id, room_id=referenced_room_id))
     await db.commit()
     await db.refresh(message)
     return message