refactor: python-dateutil 换成标准库

- contest 后台创建/编辑：dateutil.parser.parse 换成
  datetime.fromisoformat。serializer 里 start_time/end_time 声明的是
  DateTimeField，而 validate_serializer 做的是 request.data = s.data，
  即重新序列化一遍，所以视图拿到的必定是 DRF 规范化后的 ISO 8601，
  dateutil 的宽容解析能力在这里从来没用上
- AI 学情统计：relativedelta 换成 shift_months 辅助函数，复刻月末
  收缩语义（1月31日 + 1个月 = 2月28/29日）。_parse_duration 的配置
  由 delta 对象改为 rewind/advance 可调用，周档和月档统一

删除依赖：python-dateutil、six

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

uv.lock

@@ -397,7 +397,6 @@ dependencies = [
     { name = "pillow" },
     { name = "psycopg" },
     { name = "psycopg-binary" },
-    { name = "python-dateutil" },
     { name = "sentry-sdk", extra = ["django"] },
     { name = "tree-sitter" },
     { name = "tree-sitter-c" },
@@ -428,7 +427,6 @@ requires-dist = [
     { name = "pillow", specifier = ">=12.2.0,<13" },
     { name = "psycopg", specifier = ">=3.3.4,<4" },
     { name = "psycopg-binary", specifier = ">=3.3.4,<4" },
-    { name = "python-dateutil", specifier = ">=2.9.0.post0,<3" },
     { name = "sentry-sdk", extras = ["django"], specifier = ">=2.0.0,<3" },
     { name = "tree-sitter", specifier = ">=0.25.2" },
     { name = "tree-sitter-c", specifier = ">=0.24.2" },
@@ -680,18 +678,6 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/17/eb/9d89ad2d9b0ba8cd65393d434471621b98912abb10fbe1df08e480ba57b5/pydantic_core-2.46.3-graalpy312-graalpy250_312_native-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:fd35aa21299def8db7ef4fe5c4ff862941a9a158ca7b63d61e66fe67d30416b4", size = 2137657, upload-time = "2026-04-20T14:42:45.149Z" },
 ]
 
-[[package]]
-name = "python-dateutil"
-version = "2.9.0.post0"
-source = { registry = "https://pypi.org/simple" }
-dependencies = [
-    { name = "six" },
-]
-sdist = { url = "https://files.pythonhosted.org/packages/66/c0/0c8b6ad9f17a802ee498c46e004a0eb49bc148f2fd230864601a86dcf6db/python-dateutil-2.9.0.post0.tar.gz", hash = "sha256:37dd54208da7e1cd875388217d5e00ebd4179249f90fb72437e91a35459a0ad3", size = 342432, upload-time = "2024-03-01T18:36:20.211Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/ec/57/56b9bcc3c9c6a792fcbaf139543cee77261f3651ca9da0c93f5c1221264b/python_dateutil-2.9.0.post0-py2.py3-none-any.whl", hash = "sha256:a8b2bc7bffae282281c8140a97d3aa9c14da0b136dfe83f850eea9a5f7470427", size = 229892, upload-time = "2024-03-01T18:36:18.57Z" },
-]
-
 [[package]]
 name = "python-dotenv"
 version = "1.2.2"
@@ -799,15 +785,6 @@ django = [
     { name = "django" },
 ]
 
-[[package]]
-name = "six"
-version = "1.17.0"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/94/e7/b2c673351809dca68a0e064b6af791aa332cf192da575fd474ed7d6f16a2/six-1.17.0.tar.gz", hash = "sha256:ff70335d468e7eb6ec65b95b99d3a2836546063f63acc5171de367e834932a81", size = 34031, upload-time = "2024-12-04T17:35:28.174Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/b7/ce/149a00dd41f10bc29e5921b496af8b574d8413afcd5e30dfa0ed46c2cc5e/six-1.17.0-py2.py3-none-any.whl", hash = "sha256:4721f391ed90541fddacab5acf947aa0d3dc7d27b2e1e8eda2be8970586c3274", size = 11050, upload-time = "2024-12-04T17:35:26.475Z" },
-]
-
 [[package]]
 name = "sniffio"
 version = "1.3.1"