feat: Add invite codes and message replies functionality

- Introduced a new migration to add InviteCodes table and ReplyToMessageId column in Messages.
- Updated ChatHub to support replying to messages.
- Enhanced ChatService to handle message replies and validate reply targets.
- Modified UserService to implement invite-only registration mode with invite code consumption.
- Added configuration options for registration modes in appsettings.
- Created unit tests for new features including invite code registration and message reply formatting.

src/EchoHub.Tests/AsciiBannerServiceTests.cs

@@ -0,0 +1,73 @@
+using EchoHub.Core.Services;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class AsciiBannerServiceTests
+{
+    [Fact]
+    public void Render_SimpleText_ProducesFiveRows()
+    {
+        var banner = AsciiBannerService.Render("hi");
+
+        Assert.NotNull(banner);
+        Assert.Equal(5, banner!.Split('\n').Length);
+        Assert.Contains("█", banner);
+    }
+
+    [Fact]
+    public void Render_IsCaseInsensitive()
+    {
+        Assert.Equal(AsciiBannerService.Render("abc"), AsciiBannerService.Render("ABC"));
+    }
+
+    [Fact]
+    public void Render_Empty_ReturnsNull()
+    {
+        Assert.Null(AsciiBannerService.Render(""));
+        Assert.Null(AsciiBannerService.Render("   "));
+    }
+
+    [Fact]
+    public void Render_OnlyUnsupportedChars_ReturnsNull()
+    {
+        Assert.Null(AsciiBannerService.Render("🦆🦆🦆"));
+    }
+
+    [Fact]
+    public void Render_UnsupportedCharsSkipped_SupportedRemain()
+    {
+        var mixed = AsciiBannerService.Render("a🦆b");
+        var plain = AsciiBannerService.Render("ab");
+
+        Assert.Equal(plain, mixed);
+    }
+
+    [Fact]
+    public void Render_InputLongerThanCap_IsTruncatedNotRejected()
+    {
+        var banner = AsciiBannerService.Render(new string('a', AsciiBannerService.MaxInputLength + 30));
+
+        Assert.NotNull(banner);
+        // 20 glyphs of 'A' (4 cols) + 19 separators — sane width, not 50 glyphs
+        var firstRow = banner!.Split('\n')[0];
+        Assert.True(firstRow.Length <= AsciiBannerService.MaxInputLength * 6);
+    }
+
+    [Fact]
+    public void Render_DigitsAndPunctuation_Supported()
+    {
+        Assert.NotNull(AsciiBannerService.Render("42!"));
+        Assert.NotNull(AsciiBannerService.Render("v0.2"));
+    }
+
+    [Fact]
+    public void Render_FitsMessageLimits()
+    {
+        // Worst case must stay under the server's message length cap
+        var banner = AsciiBannerService.Render(new string('w', AsciiBannerService.MaxInputLength));
+
+        Assert.NotNull(banner);
+        Assert.True(banner!.Length <= EchoHub.Core.Constants.HubConstants.MaxMessageLength);
+    }
+}