fix(ovpack): restore protected scope roots

[Oldrich333]

Summary

Fixes the full-backup overwrite restore path for initialized servers by treating protected namespace roots as structural containers:

• clear children of viking://user / viking://agent instead of deleting the protected root;
• skip replaying the synthetic protected-root directory entry itself;
• retain normal replacement behavior for unprotected roots such as viking://resources.

Closes #3875.

This supersedes the approach in the closed #3876: clearing children alone still leaves restore failing when archive extraction calls mkdir("viking://user") (PERMISSION_DENIED). This change covers both phases.

Type of Change

• Bug fix

Testing

• ruff check passes for changed files.
• ruff format --check passes for changed files.
• All 19 tests in tests/misc/test_ovpack_import_policy.py pass.
• Added regression coverage for an initialized target with existing resources and protected user roots.
• End-to-end validated with the v0.4.13 Docker image: created an isolated encrypted server and account, uploaded a real restore-only OVPack through /api/v1/resources/temp_upload, restored through /api/v1/pack/restore with on_conflict=overwrite, and read the restored resource tree. The isolated container and data were removed afterward.

Safety properties

• The OVPack manifest, members, hashes, and vector payload are fully validated before any existing root is mutated (existing restore ordering is preserved).
• Invalid child records abort rather than deleting an unverified URI.
• A child deletion error propagates; restore never reports success with known stale content.

Related Issues

• Fixes [Bug]: ov restore --on-conflict overwrite fails because viking://user cannot be deleted #3875
• Supersedes closed PR fix(ovpack): restore --on-conflict overwrite clears protected scope roots via children #3876