Add ChannelManager for user-channel interactions

src/Argon.Api/Grains.Interfaces/IChannelManager.cs

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+namespace Argon.Api.Grains.Interfaces;
+
+using Entities;
+using Sfu;
+
+public interface IChannelManager : IGrainWithGuidKey
+{
+    [Alias("Join")]
+    Task<RealtimeToken> Join(Guid userId);
+
+    [Alias("Leave")]
+    Task Leave(Guid userId);
+
+    [Alias("GetChannel")]
+    Task<ChannelDto> GetChannel();
+}

src/Argon.Api/Grains.Persistence.States/UsersJoinedToChannel.cs

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+namespace Argon.Api.Grains.Persistence.States;
+
+using System.Runtime.Serialization;
+using Entities;
+using MemoryPack;
+using MessagePack;
+
+[DataContract]
+[MemoryPackable(GenerateType.VersionTolerant)]
+[MessagePackObject]
+[Serializable]
+[GenerateSerializer]
+public sealed record UsersJoinedToChannel
+{
+    public List<UsersToServerRelation> Users { get; set; } = new();
+}

src/Argon.Api/Grains/ChannelManager.cs

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+namespace Argon.Api.Grains;
+
+using Entities;
+using Interfaces;
+using Microsoft.EntityFrameworkCore;
+using Persistence.States;
+using Sfu;
+
+public class ChannelManager(
+    IArgonSelectiveForwardingUnit sfu,
+    ApplicationDbContext context,
+    [PersistentState("joinedUsers", "OrleansStorage")]
+    IPersistentState<UsersJoinedToChannel> joinedUsers
+) : Grain, IChannelManager
+{
+    public async Task<RealtimeToken> Join(Guid userId)
+    {
+        var channel = await GetChannel();
+
+        var user = (await context.Servers.Include(x => x.UsersToServerRelations)
+                .FirstAsync(x => x.Id == channel.ServerId))
+            .UsersToServerRelations.First(x => x.UserId == userId);
+
+        joinedUsers.State.Users.Add(user);
+        await joinedUsers.WriteStateAsync();
+
+        return await sfu.IssueAuthorizationTokenAsync(
+            new ArgonUserId(userId),
+            new ArgonChannelId(
+                new ArgonServerId(channel.ServerId),
+                this.GetPrimaryKey()
+            ),
+            SfuPermission.DefaultUser // TODO: sort out permissions
+        );
+    }
+
+    public Task Leave(Guid userId)
+    {
+        joinedUsers.State.Users.RemoveAll(x => x.UserId == userId);
+        return joinedUsers.WriteStateAsync();
+    }
+
+    public async Task<ChannelDto> GetChannel()
+    {
+        return await Get();
+    }
+
+    private async Task<Channel> Get()
+    {
+        return await context.Channels.FirstAsync(c => c.Id == this.GetPrimaryKey());
+    }
+}