docs: add Gitea version compatibility matrix and badge

- Create COMPATIBILITY.md with version matrix, Gitea release tracking,
  template variable reference, and compatibility verification process
- Add Gitea 1.25+ | 1.26.4 tested badge to README
- Update all 6 READMEs (EN + 5 translations) with latest tested version
  and link to COMPATIBILITY.md
- Include COMPATIBILITY.md in release artifacts

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

docs/README.zh-CN.md

@@ -78,7 +78,9 @@ cd tools && go run . dev
 
 ## 兼容性
 
-- **Gitea 1.25+**，100% 变量兼容，仅使用内置函数和官方翻译键
+- **Gitea 1.25+** — v1.25 引入的邮件模板目录结构
+- **最新测试：** Gitea 1.26.4
+- 与 Gitea 官方模板 100% 兼容 — 详见 [COMPATIBILITY.md](COMPATIBILITY.md)
 
 ## 许可证