build(backend): manage dependencies with uv

BREAKING CHANGE: apps/backend/requirements.txt is no longer the backend dependency source. Use uv sync and uv run python main.py for backend setup and startup.

apps/backend/scripts/create_admin.py

@@ -3,10 +3,7 @@
 创建管理员用户的脚本
 
 使用方法:
-    PYTHONPATH=apps python apps/backend/scripts/create_admin.py
-
-或使用虚拟环境:
-    PYTHONPATH=apps ./venv/bin/python apps/backend/scripts/create_admin.py
+    uv run python apps/backend/scripts/create_admin.py
 """
 import sys
 from pathlib import Path