M2-T13: docs wrap-up + retire frontend constraints + dependency cleanup

- README: add 前端 v2 (React SPA) section (dev/build/codegen/hosting/gates),
  update directory listing, drop stale Jinja descriptions
- architecture-overview: retire '不引入前后端分离' constraint; reflect SPA + JSON API
- roadmap: mark M2 done
- remove orphaned jinja2 dependency (recompile requirements*.txt; no other churn)
- delete empty tests/test_auth.py stub; drop dead _extract_csrf_token in test_api_data
- verified image still builds and app imports with the slimmer deps

requirements.txt

@@ -45,14 +45,10 @@ idna==3.11
     # via
     #   anyio
     #   httpx
-jinja2==3.1.6
-    # via -r requirements.in
 mako==1.3.11
     # via alembic
 markupsafe==3.0.3
-    # via
-    #   jinja2
-    #   mako
+    # via mako
 pycparser==2.23
     # via cffi
 pydantic==2.13.2