feat(api): expose async task page progress

[fenixc9]

Adds page-level progress reporting to async /tasks parsing.\n\n- Exposes progress in task status responses\n- Reports whole-task page_current/page_total/percent only\n- Supports pipeline, vlm, and hybrid backends\n- Keeps progress callback failures non-fatal\n- Preloads page totals for multi-file VLM/hybrid tasks to keep percentages stable\n\nValidation:\n- python -m py_compile mineru/cli/fast_api.py mineru/cli/common.py mineru/backend/vlm/vlm_analyze.py mineru/backend/hybrid/hybrid_analyze.py mineru/backend/pipeline/pipeline_analyze.py\n- git diff --check\n- TaskPageProgress smoke test for multi-file aggregation

[github-actions]

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[fenixc9]

I have read the CLA Document and I hereby sign the CLA

[fenixc9]

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