feat(ocr): Add parallel batch OCR processing via ThreadPoolExecutor

[echojfree]

Replace serial image-by-image OCR with batched parallel processing across all four OCR-enhanced converters (PDF, DOCX, PPTX, XLSX).

Changes

OCR Service (_ocr_service.py)

• Add max_workers parameter (default 5) to LLMVisionOCRService
• Add extract_text_batch() method using ThreadPoolExecutor for parallel I/O-bound LLM API calls
• Individual image failures captured in OCRResult.error field without interrupting the batch

PDF Converter (_pdf_converter_with_ocr.py)

• Single-pass PDF processing: collect all images from all pages first, then batch OCR, then interleave results with text lines by Y position for correct document flow
• _ocr_full_pages() now collects page renders first, then batch OCRs them (both pdfplumber and PyMuPDF fallback paths)
• Fix scanned-PDF fallback detection: regex-strip page-number headers before checking for real content
• Remove _extract_page_images() in favor of in-pass collection
• Extract _extract_text_lines_from_page() helper

DOCX Converter (_docx_converter_with_ocr.py)

• _extract_and_ocr_images(): collect all image rIds and streams first, then call extract_text_batch() once

PPTX Converter (_pptx_converter_with_ocr.py)

• Two-phase parallel strategy: Round 1: all images -> LLM caption in parallel Round 2: failed images -> OCR in parallel via extract_text_batch
• Pre-scan phase collects all image shapes across all slides (recursive _collect_image_shapes handles groups)
• Rendering phase consumes pre-computed results via cursor, maintaining identical output structure

XLSX Converter (_xlsx_converter_with_ocr.py)

• Cross-sheet collection: gather all images from all sheets before batch OCR, then distribute results by sheet
• Rename _extract_and_ocr_sheet_images -> _extract_sheet_images (collection only; OCR moved to caller)

Plugin Registration (_plugin.py)

• Accept max_workers kwarg (default 5), forward to OCR service

Tests (4 files)

• Add extract_text_batch() to all MockOCRService classes
• Update test_pdf_multipage expectation: batch-parallel path correctly extracts text+images in a single pass instead of artificially falling back to full-page OCR

Performance

Real-world test on 5-page scanned contract PDF (EMC):

• Serial (max_workers=1): 74.4s
• Parallel (max_workers=5): 24.1s
• Speedup: 3.1x

All 36 existing tests pass.

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