docs: rewrite README in English with full architecture reference

Also add itsdangerous to requirements.txt (missing implicit dep of
starlette SessionMiddleware) and a create_admin.py script for
bootstrapping a system-role user with all permissions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -18,3 +18,5 @@ __pycache__/
 *.pyc
 certbot
 web-resources
+.coverage
+password|*