worker shutdown option

[bghira]

adds --when_finished parameter for caption worker;

• stay_connected - remain online, waiting for new configs (default)
• shutdown - exit when no work is available
• post_exec_hook - run a specific command provided via --post_exec_hook as a path to an executable or script

[codecov]

Codecov Report

❌ Patch coverage is 87.34177% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/caption_flow/workers/caption.py	88.13%	7 Missing ⚠️
src/caption_flow/cli.py	83.33%	2 Missing ⚠️
src/caption_flow/models.py	87.50%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/caption_flow/models.py	95.53% <87.50%> (-0.38%)	⬇️
src/caption_flow/cli.py	46.75% <83.33%> (+1.13%)	⬆️
src/caption_flow/workers/caption.py	61.91% <88.13%> (+4.43%)	⬆️

... and 1 file with indirect coverage changes

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