chore: 将后端移动到 api/ 目录，准备合并为单仓库

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/static/admin/img/README.txt

@@ -0,0 +1,7 @@
+All icons are taken from Font Awesome (https://fontawesome.com/) project.
+The Font Awesome font is licensed under the SIL OFL 1.1:
+- https://scripts.sil.org/OFL
+
+SVG icons source: https://github.com/encharm/Font-Awesome-SVG-PNG
+Font-Awesome-SVG-PNG is licensed under the MIT license (see file license
+in current folder).