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.Storage/SqliteChannelStore.cs

@@ -37,6 +37,8 @@ public sealed class SqliteChannelStore : IResourceStore, IDisposable
             columns.CommandText = "SELECT COUNT(*) FROM pragma_table_info('seats') WHERE name='account_device_id'";
             if ((long)columns.ExecuteScalar()! == 0) Exec(_writeConn, "ALTER TABLE seats ADD COLUMN account_device_id TEXT;");
         }
+        // v7: 设置里的成员人数上限由 seats 改名为 max_members,迁移已有频道的设置 JSON(幂等)
+        Exec(_writeConn, "UPDATE channels SET settings = json_remove(json_set(settings, '$.max_members', json_extract(settings, '$.seats')), '$.seats') WHERE json_extract(settings, '$.seats') IS NOT NULL AND json_extract(settings, '$.max_members') IS NULL;");
         // Additive table: retain the running v2 database and its existing channel data.
         Exec(_writeConn, """
             CREATE TABLE IF NOT EXISTS channel_key_packages (
@@ -456,7 +458,7 @@ public sealed class SqliteChannelStore : IResourceStore, IDisposable
         public bool HasInvitation(Id16 id, string username) => Scalar("SELECT 1 FROM channel_invitations WHERE channel_id=@id AND username=@u", ("@id", id.ToArray()), ("@u", username)) is not null;
         public void PutInvitation(ChannelInvitation i) => Run("INSERT INTO channel_invitations(channel_id,username,payload,created_at) VALUES(@id,@u,@p,@t) ON CONFLICT(channel_id,username) DO UPDATE SET payload=excluded.payload,created_at=excluded.created_at", ("@id", i.ChannelId.ToArray()), ("@u", i.Username), ("@p", i.Payload), ("@t", i.CreatedAt));
         private static ChannelInvitation MapInvitation(SqliteDataReader r) => new(IdAt(r, 0), r.GetString(1), r.GetString(2), r.GetInt64(3));
-        public IReadOnlyList<ChannelInvitation> ListInvitations(Id16 id) => Many("SELECT channel_id,username,payload,created_at FROM channel_invitations WHERE channel_id=@id", MapInvitation, ("@id", id.ToArray()));
+        public IReadOnlyList<ChannelInvitation> ListInvitations(Id16 id) => Many("SELECT channel_id,username,payload,created_at FROM channel_invitations i WHERE channel_id=@id AND NOT EXISTS (SELECT 1 FROM channel_members m WHERE m.channel_id=i.channel_id AND m.username=i.username)", MapInvitation, ("@id", id.ToArray()));
         public IReadOnlyList<ChannelInvitation> ListUserInvitations(string username) => Many("SELECT i.channel_id,i.username,i.payload,i.created_at FROM channel_invitations i JOIN channels c ON c.id=i.channel_id WHERE i.username=@u AND c.expires_at>@now AND NOT EXISTS (SELECT 1 FROM channel_members m WHERE m.channel_id=i.channel_id AND m.username=i.username)", MapInvitation, ("@u", username), ("@now", DateTimeOffset.UtcNow.ToUnixTimeMilliseconds()));
         public void DeleteInvitation(Id16 id, string username) => Run("DELETE FROM channel_invitations WHERE channel_id=@id AND username=@u", ("@id", id.ToArray()), ("@u", username));
         public void PutKeyPackage(Id16 id, string username, string payload, long createdAt) => Run(
@@ -513,7 +515,7 @@ public sealed class SqliteChannelStore : IResourceStore, IDisposable
 
         // ───────── 席位 ─────────
 
-        private static Seat MapSeat(SqliteDataReader r) => new(Blob(r, 0), r.GetInt64(1), BlobOrNull(r, 2), r.IsDBNull(3) ? null : r.GetString(3), r.IsDBNull(4) ? null : r.GetString(4));
+        private static Seat MapSeat(SqliteDataReader r) => new(Blob(r, 0), r.GetInt64(1), r.IsDBNull(3) ? null : r.GetString(3), r.IsDBNull(4) ? null : r.GetString(4));
 
         public Seat? GetSeat(Id16 channelId, ReadOnlySpan<byte> endpointHash) => One(
             "SELECT endpoint_hash, joined_at, label, account_name,account_device_id FROM seats WHERE channel_id=@c AND endpoint_hash=@e",
@@ -526,17 +528,13 @@ public sealed class SqliteChannelStore : IResourceStore, IDisposable
             (int)(long)Scalar("SELECT (SELECT COUNT(*) FROM channel_members WHERE channel_id=@c) + (SELECT COUNT(*) FROM seats WHERE channel_id=@c AND account_name IS NULL)", ("@c", channelId.ToArray()))!;
 
         public void InsertSeat(Id16 channelId, Seat seat) => Run(
-            "INSERT INTO seats (channel_id, endpoint_hash, label, joined_at,account_name,account_device_id) VALUES (@c,@e,@l,@j,@a,@d)",
-            ("@c", channelId.ToArray()), ("@e", seat.EndpointHash), ("@l", seat.Label), ("@j", seat.JoinedAt), ("@a", seat.AccountName), ("@d", seat.AccountDeviceId));
+            "INSERT INTO seats (channel_id, endpoint_hash, joined_at,account_name,account_device_id) VALUES (@c,@e,@j,@a,@d)",
+            ("@c", channelId.ToArray()), ("@e", seat.EndpointHash), ("@j", seat.JoinedAt), ("@a", seat.AccountName), ("@d", seat.AccountDeviceId));
 
         public bool DeleteSeat(Id16 channelId, ReadOnlySpan<byte> endpointHash) => Run(
             "DELETE FROM seats WHERE channel_id=@c AND endpoint_hash=@e",
             ("@c", channelId.ToArray()), ("@e", endpointHash.ToArray())) > 0;
 
-        public void SetSeatLabel(Id16 channelId, ReadOnlySpan<byte> endpointHash, byte[] label) => Run(
-            "UPDATE seats SET label=@l WHERE channel_id=@c AND endpoint_hash=@e",
-            ("@l", label), ("@c", channelId.ToArray()), ("@e", endpointHash.ToArray()));
-
         public IReadOnlyList<Seat> ListSeats(Id16 channelId) => Many(
             "SELECT endpoint_hash, joined_at, label, account_name,account_device_id FROM seats WHERE channel_id=@c ORDER BY joined_at, endpoint_hash",
             MapSeat, ("@c", channelId.ToArray()));