feat: support file-backed strategy health sync token

[Pigbibi]

Summary

• allow the VPS quant monitor to read its dedicated strategy-health sync token from a root-owned regular file
• keep the token out of command arguments and the systemd unit environment
• retain the existing direct environment variable path for backward compatibility

Safety

• the publisher remains disabled unless explicitly enabled
• this only posts a read-only health dashboard snapshot; it cannot alter strategy, broker, or live configuration

Verification

• pytest -q ops/quant-monitor/tests/test_dashboard_scripts.py