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.Server/Controllers/InvitesController.cs

@@ -0,0 +1,129 @@
+using System.Security.Claims;
+using System.Security.Cryptography;
+using EchoHub.Core.DTOs;
+using EchoHub.Core.Models;
+using EchoHub.Server.Data;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.RateLimiting;
+using Microsoft.EntityFrameworkCore;
+
+namespace EchoHub.Server.Controllers;
+
+/// <summary>
+/// Invite-code management for invite-gated registration (Admin+ only).
+/// Codes live in this server's own database — there is no central service.
+/// </summary>
+[ApiController]
+[Route("api/invites")]
+[Authorize]
+[EnableRateLimiting("general")]
+public class InvitesController : ControllerBase
+{
+    private const int MaxActiveInvites = 200;
+
+    private readonly EchoHubDbContext _db;
+    private readonly ILogger<InvitesController> _logger;
+
+    public InvitesController(EchoHubDbContext db, ILogger<InvitesController> logger)
+    {
+        _db = db;
+        _logger = logger;
+    }
+
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateInviteRequest request)
+    {
+        var (caller, error) = await GetCallerAsync(ServerRole.Admin);
+        if (error is not null) return error;
+
+        var maxUses = request.MaxUses ?? 1;
+        if (maxUses is < 1 or > 1000)
+            return BadRequest(new ErrorResponse("MaxUses must be between 1 and 1000."));
+
+        if (request.ExpiresInHours is < 1 or > 24 * 365)
+            return BadRequest(new ErrorResponse("ExpiresInHours must be between 1 and 8760."));
+
+        if (await _db.InviteCodes.CountAsync(i => i.UseCount < i.MaxUses) >= MaxActiveInvites)
+            return BadRequest(new ErrorResponse($"Too many active invites (max {MaxActiveInvites}). Revoke unused ones first."));
+
+        var invite = new InviteCode
+        {
+            Id = Guid.NewGuid(),
+            Code = GenerateCode(),
+            CreatedByUserId = caller!.Id,
+            CreatedByUsername = caller.Username,
+            CreatedAt = DateTimeOffset.UtcNow,
+            ExpiresAt = request.ExpiresInHours is { } hours ? DateTimeOffset.UtcNow.AddHours(hours) : null,
+            MaxUses = maxUses,
+        };
+
+        _db.InviteCodes.Add(invite);
+        await _db.SaveChangesAsync();
+
+        _logger.LogInformation("Invite code created by {User} (uses: {MaxUses}, expires: {Expires})",
+            caller.Username, invite.MaxUses, invite.ExpiresAt?.ToString("u") ?? "never");
+
+        return Ok(ToDto(invite));
+    }
+
+    [HttpGet]
+    public async Task<IActionResult> List()
+    {
+        var (_, error) = await GetCallerAsync(ServerRole.Admin);
+        if (error is not null) return error;
+
+        var invites = await _db.InviteCodes
+            .OrderByDescending(i => i.CreatedAt)
+            .ToListAsync();
+
+        return Ok(invites.Select(ToDto).ToList());
+    }
+
+    [HttpDelete("{code}")]
+    public async Task<IActionResult> Revoke(string code)
+    {
+        var (caller, error) = await GetCallerAsync(ServerRole.Admin);
+        if (error is not null) return error;
+
+        var normalized = code.Trim().ToUpperInvariant();
+        var invite = await _db.InviteCodes.FirstOrDefaultAsync(i => i.Code == normalized);
+        if (invite is null)
+            return NotFound(new ErrorResponse("Invite code not found."));
+
+        _db.InviteCodes.Remove(invite);
+        await _db.SaveChangesAsync();
+
+        _logger.LogInformation("Invite code revoked by {User}", caller!.Username);
+        return Ok();
+    }
+
+    /// <summary>Unguessable, unambiguous code like "K7QM-3XPF" (no 0/O/1/I).</summary>
+    private static string GenerateCode()
+    {
+        const string alphabet = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789";
+        Span<char> chars = stackalloc char[8];
+        for (var i = 0; i < chars.Length; i++)
+            chars[i] = alphabet[RandomNumberGenerator.GetInt32(alphabet.Length)];
+        return $"{new string(chars[..4])}-{new string(chars[4..])}";
+    }
+
+    private static InviteDto ToDto(InviteCode i) =>
+        new(i.Code, i.CreatedByUsername, i.CreatedAt, i.ExpiresAt, i.MaxUses, i.UseCount);
+
+    private async Task<(User? Caller, IActionResult? Error)> GetCallerAsync(ServerRole minimumRole)
+    {
+        var userIdClaim = User.FindFirstValue(ClaimTypes.NameIdentifier);
+        if (userIdClaim is null)
+            return (null, Unauthorized(new ErrorResponse("Authentication required.")));
+
+        var caller = await _db.Users.FindAsync(Guid.Parse(userIdClaim));
+        if (caller is null)
+            return (null, Unauthorized(new ErrorResponse("User not found.")));
+
+        if (caller.Role < minimumRole)
+            return (null, StatusCode(403, new ErrorResponse($"Requires {minimumRole} role or higher.")));
+
+        return (caller, null);
+    }
+}