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/css/unusable_password_field.css

@@ -0,0 +1,19 @@
+/* Hide warnings fields if usable password is selected */
+form:has(#id_usable_password input[value="true"]:checked) .messagelist {
+    display: none;
+}
+
+/* Hide password fields if unusable password is selected */
+form:has(#id_usable_password input[value="false"]:checked) .field-password1,
+form:has(#id_usable_password input[value="false"]:checked) .field-password2 {
+    display: none;
+}
+
+/* Select appropriate submit button */
+form:has(#id_usable_password input[value="true"]:checked) input[type="submit"].unset-password {
+    display: none;
+}
+
+form:has(#id_usable_password input[value="false"]:checked) input[type="submit"].set-password {
+    display: none;
+}