shuriken-sync: publish twice daily, only when content changed

Colliding with the nightly generation CronJob's flock at the top of
some hours caused an intermittent EBADF failure. Move sync to two
fixed daytime ticks (10:00/18:00 Europe/Sofia) clear of the 04:00
generation run, and replace the mtime-based "did a generation
complete" check with a per-site content diff (image_count/
total_size_bytes vs a backed-up status.json from the last publish),
so a tick with no real content change is a no-op instead of an
unconditional publish.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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