feat(api)!: fold devices into users and report member presence

Contract 7.0.0 (breaking). Devices become an attribute of the account;
members are managed per user, and only unauthenticated guests keep a
per-terminal registration.

- rename seats -> max_members, seats_used -> members_used,
  seats_full -> members_full, SeatJoined -> JoinResult, the `seats`
  stream control message -> `members`, and POST /channels/{id}/seats ->
  POST /channels/{id}/join (stored channel settings are migrated on start)
- drop per-terminal encrypted device labels and the seat list/release
  endpoints; add GET/DELETE /channels/{id}/guests for guest terminals
- authenticate account members by account session only: the terminal key
  is optional for logged-in accounts, no seat row is written for them,
  and gated-download grants are counted per user (per terminal for guests)
- GET /channels/{id}/members now returns online, devices and
  online_devices per account; presence is tracked in memory from active
  WebSocket/SSE subscriptions and is not pushed over the stream
- owner invitation list no longer returns accounts that already joined

server/tests/WebDrop.Core.Tests/TestEnv.cs

@@ -14,10 +14,10 @@ public sealed class FakeClock : IClock
 public sealed class RecordingEvents : IChannelEvents
 {
     public List<(Id16 Channel, long Seq)> Advances { get; } = [];
-    public int Settings, Seats, Deleted;
+    public int Settings, Members, Deleted;
     public void Advanced(Id16 channelId, long seq) { lock (Advances) Advances.Add((channelId, seq)); }
     public void SettingsChanged(Id16 channelId) => Interlocked.Increment(ref Settings);
-    public void SeatsChanged(Id16 channelId) => Interlocked.Increment(ref Seats);
+    public void MembersChanged(Id16 channelId) => Interlocked.Increment(ref Members);
     public void ChannelDeleted(Id16 channelId) => Interlocked.Increment(ref Deleted);
 }