feat(admin): add user management, quotas, and stats dashboard to admin panel

- Dashboard tab: total users, active users, admins, storage usage, quota warnings
- User management tab: list, edit role, activate/deactivate, update quota, delete
- Self-protection: cannot delete/deactivate/demote yourself
- Paginated user listing (default 50 per page)
- Efficient single-query dashboard stats with direct SQL aggregation
- Quota modal with GB/MB/TB unit selector
- Backend: added methods to UserStoragePort, UserRepository, PgRepository
- Backend: added 7 admin methods to AuthApplicationService
- Backend: added 5 new DTOs for admin operations
- Backend: 7 new endpoints under /api/admin/

Files modified:
- src/application/ports/auth_ports.rs (4 new UserStoragePort methods)
- src/domain/repositories/user_repository.rs (StorageStats + 3 methods)
- src/infrastructure/repositories/pg/user_pg_repository.rs (implementations)
- src/application/services/auth_application_service.rs (admin methods)
- src/application/dtos/settings_dto.rs (5 new DTOs)
- src/interfaces/api/handlers/admin_handler.rs (7 new endpoints)
- static/admin.html (complete UI with 3 tabs: Dashboard, Users, OIDC)

src/domain/repositories/user_repository.rs

@@ -94,4 +94,24 @@ pub trait UserRepository: Send + Sync + 'static {
 
     /// Finds a user by OIDC provider + subject pair
     async fn get_user_by_oidc_subject(&self, provider: &str, subject: &str) -> UserRepositoryResult<User>;
+
+    /// Actualiza la cuota de almacenamiento de un usuario
+    async fn update_storage_quota(&self, user_id: &str, quota_bytes: i64) -> UserRepositoryResult<()>;
+
+    /// Cuenta el número total de usuarios
+    async fn count_users(&self) -> UserRepositoryResult<i64>;
+
+    /// Obtiene estadísticas de almacenamiento agregadas
+    async fn get_storage_stats(&self) -> UserRepositoryResult<StorageStats>;
+}
+
+/// Estadísticas de almacenamiento agregadas
+#[derive(Debug, Clone)]
+pub struct StorageStats {
+    pub total_users: i64,
+    pub active_users: i64,
+    pub total_quota_bytes: i64,
+    pub total_used_bytes: i64,
+    pub users_over_80_percent: i64,
+    pub users_over_quota: i64,
 }