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/PrimitivesTests.cs

@@ -155,9 +155,9 @@ public class PolicyTests
     [Fact]
     public void SettingsPatchOnlyChangesProvidedFields()
     {
-        var before = new ChannelSettings { Seats = 2, TtlSeconds = 100 };
-        var after = new ChannelSettingsPatch { Seats = 5 }.ApplyTo(before);
-        Assert.Equal(5, after.Seats);
+        var before = new ChannelSettings { MaxMembers = 2, TtlSeconds = 100 };
+        var after = new ChannelSettingsPatch { MaxMembers = 5 }.ApplyTo(before);
+        Assert.Equal(5, after.MaxMembers);
         Assert.Equal(100, after.TtlSeconds);
     }
 
@@ -169,7 +169,7 @@ public class PolicyTests
     [InlineData(2, 86400, 0)]
     public void SettingsValidationRejectsOutOfRange(int seats, int ttl, int maxItems)
     {
-        var s = new ChannelSettings { Seats = seats, TtlSeconds = ttl, MaxItems = maxItems };
+        var s = new ChannelSettings { MaxMembers = seats, TtlSeconds = ttl, MaxItems = maxItems };
         var ex = Assert.Throws<WebDropException>(() => s.Validate(new ServerLimits()));
         Assert.Equal(ErrorCode.BadRequest, ex.Code);
     }