Add foundational user authentication and registration system

Introduces schemas for user management, token handling, and password hashing. Implements routes for user registration, login, token refresh, and user info retrieval. Sets up authentication dependencies and integrates the API router with the application.

backend/app/api/main.py

@@ -0,0 +1,6 @@
+from fastapi import APIRouter
+
+from app.api.routes import auth
+
+api_router = APIRouter()
+api_router.include_router(auth.router, tags=["auth"])