Initial vault-backup project

- R2 백업 스크립트 (Raft 스냅샷 + fallback)
- 경로 기반 백업 스크립트
- 환경변수 템플릿
- README 문서

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

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+# Vault Backup R2 Configuration
+# Copy to vault-backup-r2.env and fill in values
+
+# Vault Configuration
+VAULT_ADDR=https://vault.anvil.it.com
+VAULT_TOKEN=hvs.xxxxxxxxxxxxx
+
+# Cloudflare R2 Configuration
+R2_ACCOUNT_ID=your-cloudflare-account-id
+R2_ACCESS_KEY=your-r2-access-key-id
+R2_SECRET_KEY=your-r2-secret-access-key
+R2_BUCKET=vault-backup
+
+# Backup Settings
+BACKUP_DIR=/tmp/vault-backups
+RETENTION_DAYS=30