feat(frontend): discord registration/login/linking

also moves the registration form found on the mastodon callback page
into a component so we're not repeating the same code for every auth method

Foxnouns.Backend/Controllers/Authentication/AuthController.cs

@@ -5,6 +5,7 @@ using Foxnouns.Backend.Database.Models;
 using Foxnouns.Backend.Extensions;
 using Foxnouns.Backend.Middleware;
 using Foxnouns.Backend.Services;
+using Foxnouns.Backend.Utils;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.EntityFrameworkCore;
 using Newtonsoft.Json;
@@ -56,9 +57,10 @@ public class AuthController(
 
     public record AddOauthAccountResponse(
         Snowflake Id,
-        AuthType Type,
+        [property: JsonConverter(typeof(ScreamingSnakeCaseEnumConverter))] AuthType Type,
         string RemoteId,
-        string? RemoteUsername
+        [property: JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
+            string? RemoteUsername
     );
 
     public record OauthRegisterRequest(string Ticket, string Username);

Foxnouns.Backend/Controllers/MetaController.cs

@@ -27,7 +27,8 @@ public class MetaController : ApiControllerBase
                 new Limits(
                     MemberCount: MembersController.MaxMemberCount,
                     BioLength: ValidationUtils.MaxBioLength,
-                    CustomPreferences: ValidationUtils.MaxCustomPreferences
+                    CustomPreferences: ValidationUtils.MaxCustomPreferences,
+                    MaxAuthMethods: AuthUtils.MaxAuthMethodsPerType
                 )
             )
         );
@@ -49,5 +50,10 @@ public class MetaController : ApiControllerBase
     private record UserInfo(int Total, int ActiveMonth, int ActiveWeek, int ActiveDay);
 
     // All limits that the frontend should know about (for UI purposes)
-    private record Limits(int MemberCount, int BioLength, int CustomPreferences);
+    private record Limits(
+        int MemberCount,
+        int BioLength,
+        int CustomPreferences,
+        int MaxAuthMethods
+    );
 }

Foxnouns.Backend/Services/Auth/AuthService.cs

@@ -223,6 +223,15 @@ public class AuthService(
     {
         AssertValidAuthType(authType, null);
 
+        // This is already checked when
+        var currentCount = await db
+            .AuthMethods.Where(m => m.UserId == userId && m.AuthType == authType)
+            .CountAsync(ct);
+        if (currentCount >= AuthUtils.MaxAuthMethodsPerType)
+            throw new ApiError.BadRequest(
+                "Too many linked accounts of this type, maximum of 3 per account."
+            );
+
         var authMethod = new AuthMethod
         {
             Id = snowflakeGenerator.GenerateSnowflake(),