Make caption refusal markers configurable

[bghira]

Replace the OpenAI-compatible worker's fixed refusal matching with configurable markers.

• inference.refusal_markers sets the shared job-level list
• openai_compatible.refusal_markers provides a worker-local override
• an empty list disables text refusal matching
• markers are validated, normalized, deduplicated, and reloadable
• literal image text such as “I can't stay at home” can be accepted without weakening unrelated jobs

Validation:

• 494 tests passed
• 100% changed-line coverage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/caption_flow/utils/vllm_config.py	93.33% <ø> (ø)
src/caption_flow/workers/openai_compatible.py	90.86% <100.00%> (+0.33%)	⬆️

... and 3 files with indirect coverage changes

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