Fix #433 Prevent per-page hangs & avoid killing job on maxbackoff

[akshan-main]

Closes #433

Changes proposed in this pull request:

• apost() now takes a timeout_s param and wraps the entire network path in asyncio.timeout(), so a stalled server cant block forever
• When max backoff is exhausted, we return None instead of sys.exit(1) - the existing fallback path (make_fallback_result) handles it from there, so the rest of the PDF still gets processed
• New --request_timeout_s CLI flag (default 120s) to control per-request timeout

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[akshan-main]

@jakep-allenai