Add headless Garmin background sync (systemd timer)

New `bincio sync-garmin` command mirrors sync-strava: discovers all users
with garmin_creds.json, refreshes cached garth OAuth2 session, imports new
activities, and optionally POSTs to the rebuild endpoint.

systemd timer fires every 3h offset by 1h30m from Strava to avoid
simultaneous rebuilds. Status written to _garmin_sync_status.json per user.

bincio/cli.py

@@ -20,6 +20,7 @@ from bincio.serve.cli import serve              # noqa: E402
 from bincio.dev import dev                      # noqa: E402
 from bincio.reextract_cmd import reextract_originals  # noqa: E402
 from bincio.sync_strava import sync_strava_cmd        # noqa: E402
+from bincio.sync_garmin import sync_garmin_cmd        # noqa: E402
 from bincio.segments.cli import segments_group        # noqa: E402
 
 
@@ -49,4 +50,5 @@ main.add_command(serve)
 main.add_command(dev)
 main.add_command(reextract_originals)
 main.add_command(sync_strava_cmd)
+main.add_command(sync_garmin_cmd)
 main.add_command(segments_group)