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>

backend/app/services/admin_service.py

@@ -93,9 +93,14 @@ async def set_user_site_admin(
 
 
 async def list_rooms_admin(db: AsyncSession) -> list[tuple[Room, int]]:
+    # #52: DMs are fully private, not just unlisted -- excluded here rather
+    # than merely omitted from the response, so there's no admin-portal
+    # surface (this list, or the audit log via anything that touches this
+    # query) that reveals a DM even exists between two users.
     result = await db.execute(
         select(Room, func.count(RoomMembership.user_id))
         .outerjoin(RoomMembership, RoomMembership.room_id == Room.id)
+        .where(Room.is_dm.is_(False))
         .group_by(Room.id)
         .order_by(Room.created_at)
     )