wire repository and utility bindings

AppServiceProvider binds Clock -> SystemClock, TokenGenerator ->
RandomTokenGenerator, PasswordHasher -> BcryptPasswordHasher.
new RepositoryServiceProvider binds UserRepository ->
EloquentUserRepository, SessionRepository ->
EloquentSessionRepository. bootstrap/providers.php registers
both. verified container resolves: app(Clock::class) returns
SystemClock. migrations create users + sessions tables with
proper unique/foreign-key/index constraints (sqlite roundtrip
confirmed).

backend/bootstrap/providers.php

@@ -1,7 +1,9 @@
 <?php
 
 use App\Providers\AppServiceProvider;
+use App\Providers\RepositoryServiceProvider;
 
 return [
     AppServiceProvider::class,
+    RepositoryServiceProvider::class,
 ];