The initial 'installing' record MUST exist for all subsequent status
updates to succeed. Previously this was fire-and-forget with no retry,
so timeouts/503s silently dropped ~50% of installations.
Both post_to_api (LXC) and post_to_api_vm now retry up to 3 times
with 1s delay between attempts. Also captures HTTP response code to
detect failures instead of using curl -f (silent fail).
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