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.ko.md

@@ -55,3 +55,12 @@ cd tools && go run . dev
 # → http://localhost:3456
 ```
 
+## 호환성
+
+- **Gitea 1.25+** — v1.25에서 도입된 메일 템플릿 디렉토리 구조 사용
+- **최신 테스트:** Gitea 1.26.4
+- Gitea 공식 템플릿과 100% 호환 — 자세한 내용은 [COMPATIBILITY.md](../COMPATIBILITY.md) 참조
+
+## 라이선스
+
+MIT — [LICENSE](../LICENSE).