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>

frontend/src/components/MessageList.tsx

@@ -64,11 +64,12 @@ interface MessageListProps {
   roomId: string
   messages: (Message | ChatMessageEnvelope)[]
   members: RoomMember[]
+  myRooms: Map<string, string>
   onEdit: (messageId: string, content: string) => void
   onReact: (messageId: string, emoji: string) => void
 }
 
-export function MessageList({ roomId, messages, members, onEdit, onReact }: MessageListProps) {
+export function MessageList({ roomId, messages, members, myRooms, onEdit, onReact }: MessageListProps) {
   const { user } = useAuth()
   const bottomRef = useRef<HTMLDivElement>(null)
   const [editingId, setEditingId] = useState<string | null>(null)
@@ -168,7 +169,7 @@ export function MessageList({ roomId, messages, members, onEdit, onReact }: Mess
                   )}
                   {msg.content && (
                     <div className="message-text">
-                      <MessageContent content={msg.content} memberUsernames={memberUsernames} />
+                      <MessageContent content={msg.content} memberUsernames={memberUsernames} myRooms={myRooms} />
                       {msg.edited_at && <span className="message-edited"> (edited)</span>}
                     </div>
                   )}