feat: add username-password login flow via authentik token endpoint

backend/app/main.py

@@ -2,6 +2,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.api.admin import router as admin_router
+from app.api.auth import router as auth_router
 from app.api.internal import router as internal_router
 from app.api.me import router as me_router
 from app.core.config import get_settings
@@ -26,3 +27,4 @@ def healthz() -> dict[str, str]:
 app.include_router(internal_router)
 app.include_router(admin_router)
 app.include_router(me_router)
+app.include_router(auth_router)