Add synchronous startup verification after Popen() — sleep 0.5s and poll
the process before returning to the caller. If SatDump exits immediately
(missing device, bad args), raise RuntimeError with the actual error
message instead of returning status: 'started'. Keep a shorter (2s) async
backup check for slower failures.
Also fix --source_id handling: omit the flag entirely when no serial number
is found instead of passing "0" which SatDump may reject. Change start()
and start_from_file() to return (bool, str|None) tuples so error messages
propagate through to the HTTP response.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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