Add direct messages (#52)

A DM is a Room with a new is_dm flag, not a separate model -- reuses
all the membership/message/WS plumbing Room already has instead of
duplicating it. The room's `name` (still required + globally unique)
is an internal, never-displayed token derived deterministically from
the two participants' sorted user IDs (dm_room_name), which makes
find-or-create a single indexed lookup and gets free race-condition
safety from the existing unique constraint -- a concurrent double-
start from both people just hits the same IntegrityError->retry-as-
lookup path create_room already established.

Both participants get the plain 'member' role (no owner/admin
distinction makes sense for a 1:1 DM), which incidentally reuses
every existing role gate to block add-member, room-settings edits,
and join-via-browse on a DM for free. update_room also gets an
explicit is_dm guard independent of that, since renaming a DM isn't
just a privacy concern -- it would silently corrupt the find-or-create
invariant. DMs are excluded from both Browse Rooms and the admin
portal's room listing (fully private, per scope).

GET /api/rooms/mine precomputes each DM's other participant (name,
avatar, presence) as dm_partner in one batched query, so the sidebar
can render a DM row without a fetch per row. Frontend: a new "Direct
Messages" sidebar section (searchable by partner name, not the
internal room name), clicking someone in the People list starts or
resumes a DM, and the chat header/composer/RoomInfoPanel all render
the partner's identity instead of a room name where it's a DM.

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

frontend/src/components/ChatPane.tsx

@@ -224,8 +224,16 @@ export function ChatPane({
           </button>
         )}
         <div className="chat-pane-title-block">
-          <div className="chat-pane-title">#{room.name}</div>
-          <div className="chat-pane-subtitle">{members.length} member{members.length === 1 ? '' : 's'}</div>
+          <div className="chat-pane-title">
+            {room.dm_partner ? room.dm_partner.display_name || room.dm_partner.username : `#${room.name}`}
+          </div>
+          <div className="chat-pane-subtitle">
+            {room.dm_partner
+              ? room.dm_partner.status === 'online'
+                ? 'Online'
+                : 'Offline'
+              : `${members.length} member${members.length === 1 ? '' : 's'}`}
+          </div>
         </div>
         <button
           type="button"
@@ -259,7 +267,8 @@ export function ChatPane({
       />
       <Composer
         roomId={room.id}
-        roomName={room.name}
+        roomName={room.dm_partner ? room.dm_partner.display_name || room.dm_partner.username : room.name}
+        isDm={room.is_dm}
         members={members}
         rooms={rooms}
         disabled={!connected}