implement auth controller and routes

Wires AuthController (signup, confirmEmail, login, me, logout)
to the existing auth use cases. Routes mounted under /api with
AuthMiddleware on logout/me. RepositoryServiceProvider gains
EmailConfirmationToken and Post bindings; AppServiceProvider
binds the Emailer/EmailFactory and constructs SignupUser with
the configured from-address.

backend/routes/api.php

@@ -1,6 +1,17 @@
 <?php
 
-// Route registrations land here as the auth, user, post, and comment
-// domains come online. Empty for now - the skeleton commit only wires
-// up the file so bootstrap/app.php's withRouting(api: ...) has a valid
-// target.
+use App\Controllers\AuthController;
+use App\Http\Middleware\AuthMiddleware;
+use Illuminate\Support\Facades\Route;
+
+Route::get('/', function () {
+    return '';
+});
+
+Route::post('/signup', [AuthController::class, 'signup']);
+Route::post('/confirm-email', [AuthController::class, 'confirmEmail']);
+Route::post('/login', [AuthController::class, 'login']);
+Route::post('/logout', [AuthController::class, 'logout'])
+    ->middleware(AuthMiddleware::class);
+Route::get('/me', [AuthController::class, 'me'])
+    ->middleware(AuthMiddleware::class);