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/room_reference_service.py

@@ -0,0 +1,31 @@
+import re
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import Room, RoomMembership
+from app.services.mention_service import strip_code_spans
+
+ROOM_REFERENCE_PATTERN = re.compile(r"#([a-zA-Z0-9_.-]+)")
+
+
+async def extract_referenced_room_ids(
+    db: AsyncSession, sender_id: uuid.UUID, content: str
+) -> set[uuid.UUID]:
+    """Resolves `#roomname` tokens in `content` against rooms the *sender*
+    is a member of -- deliberately not the room the message is being sent
+    in (the whole point is referencing a *different* room), and not open to
+    arbitrary site rooms either (#47: referencing a private room the sender
+    isn't in would leak its existence to anyone reading the message, even
+    though they aren't in it either)."""
+    room_names = set(ROOM_REFERENCE_PATTERN.findall(strip_code_spans(content)))
+    if not room_names:
+        return set()
+
+    result = await db.execute(
+        select(Room.id)
+        .join(RoomMembership, RoomMembership.room_id == Room.id)
+        .where(RoomMembership.user_id == sender_id, Room.name.in_(room_names))
+    )
+    return {row[0] for row in result.all()}