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/src/WebDrop.Core/Errors.cs

@@ -6,7 +6,7 @@ public enum ErrorCode
     Unauthorized,
     Forbidden,
     NotJoined,
-    SeatsFull,
+    MembersFull,
     ModeMismatch,
     NotFound,
     Conflict,
@@ -47,7 +47,7 @@ public static class ErrorCodes
         ErrorCode.Unauthorized => "unauthorized",
         ErrorCode.Forbidden => "forbidden",
         ErrorCode.NotJoined => "not_joined",
-        ErrorCode.SeatsFull => "seats_full",
+        ErrorCode.MembersFull => "members_full",
         ErrorCode.ModeMismatch => "mode_mismatch",
         ErrorCode.NotFound => "not_found",
         ErrorCode.Conflict => "conflict",
@@ -70,7 +70,7 @@ public static class ErrorCodes
     {
         ErrorCode.BadRequest => 400,
         ErrorCode.Unauthorized => 401,
-        ErrorCode.Forbidden or ErrorCode.NotJoined or ErrorCode.SeatsFull or ErrorCode.ModeMismatch => 403,
+        ErrorCode.Forbidden or ErrorCode.NotJoined or ErrorCode.MembersFull or ErrorCode.ModeMismatch => 403,
         ErrorCode.NotFound => 404,
         ErrorCode.Conflict => 409,
         ErrorCode.Sealed => 409,