[STACKED on #873] fix(kms): persist private keys owner-only

[kvinwang]

Problem

KMS wrote private-key files with permissions derived from the generic safe_write() path. A permissive environment could leave CA or service private keys readable by other local users.

Fix

The upstream safe-write crate now provides:

safe_write_with_mode(path, content, mode)

Version 0.1.3 has been published to crates.io and tagged as v0.1.3 upstream. KMS now uses that API with mode 0o600 for:

• temporary CA private key;
• root CA private key;
• RPC private key;
• K256 private key.

This keeps atomic-write behavior centralized in its owning crate instead of duplicating it in KMS.

Changed paths

• dstack/Cargo.toml
• dstack/Cargo.lock
• dstack/kms/src/onboard_service.rs

Dependency and merge order

• Parent PR: [STACKED on #863] fix(kms): make Bootstrap one-time #873 (merged)
• GitHub base branch: codex/fix-kms-bootstrap-once

Verification

• Upstream safe-write: all unit and documentation tests passed.
• Upstream cargo publish --dry-run: passed.
• safe-write v0.1.3: published successfully to crates.io.
• cargo test -p dstack-kms onboard_service::tests --no-fail-fast: passed.
• cargo fmt --all -- --check: passed.
• git diff --check: passed.

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