fix: enforce unique usernames (#1628)

* fix: enforce unique usernames

Registration checked for a taken name, hashed the password, then
inserted, with nothing in the schema enforcing uniqueness. Concurrent
sign-ups could create two accounts with one name, after which login,
password reset, proxy login and admin lookups by username picked
between them arbitrarily.

Add a unique index on users.username for every dialect. Existing
duplicates are renamed (later rows get a -<id> suffix, the oldest keeps
the name) rather than deleted. Registration and admin user creation now
return the same 409 when the insert loses the race.

* fix: avoid username collisions during migration

src/backend/database/routes/user-admin-routes.ts

@@ -17,9 +17,10 @@ import {
   listUserSecondFactors,
   resetUserSecondFactors,
 } from "../../auth/second-factor-admin.js";
-import type {
-  UserRecord,
-  UserRepository,
+import {
+  isUsernameTaken,
+  type UserRecord,
+  type UserRepository,
 } from "../repositories/user-repository.js";
 
 function isNonEmptyString(val: unknown): val is string {
@@ -541,6 +542,9 @@ export function registerUserAdminRoutes(
         toast: { type: "success", message: `User created: ${username}` },
       });
     } catch (err) {
+      if (isUsernameTaken(err)) {
+        return res.status(409).json({ error: "Username already exists" });
+      }
       authLogger.error("Failed to admin-create user", err);
       res.status(500).json({ error: "Failed to create user" });
     }