fix(security): Fix DOMPDF RCE and customer email sanitization

- Disable isPhpEnabled in DOMPDF to prevent RCE via embedded PHP in HTML
- Disable isRemoteEnabled to prevent SSRF attacks
- Add email validation and sanitization in CSV import (FILTER_SANITIZE_EMAIL, FILTER_VALIDATE_EMAIL)
- Reject invalid email formats during customer import
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f