feat: panel admin - gestion utilisateurs (#16)

- Route /api/admin avec middleware require_admin
- Liste utilisateurs avec statut, role, dates
- Actions: activer/desactiver, promouvoir/rétrograder admin, supprimer
- Dashboard stats (utilisateurs, téléchargements)
- Template admin_panel.html avec table responsive
- Champ is_admin ajoute au modele User
- Migration automatique colonne is_admin
- Protection: impossible de modifier son propre compte

Closes #16

app/routers/__init__.py

@@ -13,6 +13,7 @@ from app.routers.router_player import router as player_router
 from .router_static import router as static_router
 from .router_root import router as root_router
 from .router_settings import router as settings_router
+from .router_admin import router as admin_router
 
 __all__ = [
     "auth_router",
@@ -26,5 +27,6 @@ __all__ = [
     "static_router",
     "root_router",
     "settings_router",
+    "admin_router",
 ]