docs(api): 📝 Document API parameter to change username

docs/api/mastodon.md

@@ -62,3 +62,10 @@ An extra attribute has been adding to all returned account objects:
 ### `roles`
 
 An array of roles from [Lysand Roles](./roles.md).
+
+### `/api/v1/accounts/update_credentials`
+
+The `username` parameter can now (optionally) be set to change the user's handle.
+
+> [!WARNING]
+> Clients should indicate to users that changing their handle will break existing links to their profile. This is reversible, but the old handle will be available for anyone to claim.

server/api/api/v1/accounts/update_credentials/index.ts

@@ -3,7 +3,7 @@ import { errorResponse, jsonResponse } from "@/response";
 import { sanitizedHtmlStrip } from "@/sanitization";
 import { zValidator } from "@hono/zod-validator";
 import { config } from "config-manager";
-import { and, eq } from "drizzle-orm";
+import { and, eq, isNull } from "drizzle-orm";
 import type { Hono } from "hono";
 import ISO6391 from "iso-639-1";
 import { MediaBackend } from "media-manager";
@@ -12,7 +12,7 @@ import { getUrl } from "~/database/entities/attachment";
 import { parseEmojis } from "~/database/entities/emoji";
 import { contentToHtml } from "~/database/entities/status";
 import { db } from "~/drizzle/db";
-import { EmojiToUser, RolePermissions } from "~/drizzle/schema";
+import { EmojiToUser, RolePermissions, Users } from "~/drizzle/schema";
 import type { Emoji } from "~/packages/database-interface/emoji";
 import { User } from "~/packages/database-interface/user";
 
@@ -191,6 +191,15 @@ export default (app: Hono) =>
             }
 
             if (username) {
+                // Check if username is already taken
+                const existingUser = await User.fromSql(
+                    and(isNull(Users.instanceId), eq(Users.username, username)),
+                );
+
+                if (existingUser) {
+                    return errorResponse("Username is already taken", 400);
+                }
+
                 self.username = username;
             }