Replace active S3 sync with encrypted WebDAV synchronization and independent backups. Support explicit empty passwords and unauthenticated MongoDB, Redis, HTTP and HTTPS targets. Include migration, design and validation documentation.
Validation: build and go vet passed; Playwright 113 passed with 4 existing failures. Full Go run reproduced the existing cluster discovery test failure. Preserve unrelated SSH fingerprint changes outside this commit.
曾志威