feat(autopilot-manager): set system time from FC GNSS time when NTP is not synced

[dakejahl]

Summary

Set the local system clock from the flight controller's GNSS-derived UTC time (SYSTEM_TIME) when the clock has no NTP sync.

Problem

A Jetson/Pi with no network has no NTP source and no battery-backed RTC, so its clock is wrong from boot until the network appears. An FC with GNSS already knows UTC and streams it in SYSTEM_TIME, but nothing consumed it — time sync only worked in the companion→FC direction.

Solution

autopilot-manager requests SYSTEM_TIME at 1 Hz (re-requested after 30 s of silence, since PX4 mutes the stream while its clock is unset and a reboot drops the configured interval) and steps the clock via clock_settime when the FC time is valid, the kernel reports no NTP sync, and the clocks disagree by more than 2 s (absorbs transport latency). The two directions cannot fight: FC→local runs only while unsynced, and clock_settime does not raise the kernel NTP-synced flag that gates the existing local→FC push. The systemd units grant AmbientCapabilities=CAP_SYS_TIME, and the message loop's interval timers and heartbeat staleness checks move to time.monotonic() so the service's own clock steps cannot stall or misfire them.