fix: OCR Track reflow PDF and translation with image text filtering

- Add OCR Track support for reflow PDF generation using raw_ocr_regions.json
- Add OCR Track translation extraction from raw_ocr_regions instead of elements
- Add raw_ocr_translations output format for OCR Track documents
- Add exclusion zone filtering to remove text overlapping with images
- Update API validation to accept both translations and raw_ocr_translations
- Add page_number field to TranslatedItem for proper tracking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/archive/2025-12-12-fix-ocr-track-reflow-pdf/proposal.md

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+# Change: Fix OCR Track Reflow PDF
+
+## Why
+
+The OCR Track reflow PDF generation is missing most content because:
+
+1. PP-StructureV3 extracts tables as elements but stores `content: ""` (empty string) instead of structured `content.cells` data
+2. The `generate_reflow_pdf` method expects `content.cells` for tables, so tables are skipped
+3. Table text exists in `raw_ocr_regions.json` (59 text blocks) but is not used by reflow PDF generation
+4. This causes significant content loss - only 6 text elements vs 59 raw OCR regions
+
+The Layout PDF works correctly because it uses `raw_ocr_regions.json` via Simple Text Positioning mode, bypassing the need for structured table data.
+
+## What Changes
+
+### Reflow PDF Generation for OCR Track
+
+Modify `generate_reflow_pdf` to use `raw_ocr_regions.json` as the primary text source for OCR Track documents:
+
+1. **Detect processing track** from JSON metadata
+2. **For OCR Track**: Load `raw_ocr_regions.json` and render all text blocks in reading order
+3. **For Direct Track**: Continue using `content.cells` for tables (already works)
+4. **Images/Charts**: Continue using `content.saved_path` from elements (works for both tracks)
+
+### Data Flow
+
+**OCR Track Reflow PDF (NEW):**
+```
+raw_ocr_regions.json (59 text blocks)
+  + scan_result.json (images/charts only)
+  → Sort by Y coordinate (reading order)
+  → Render text paragraphs + images
+```
+
+**Direct Track Reflow PDF (UNCHANGED):**
+```
+*_result.json (elements with content.cells)
+  → Render tables, text, images in order
+```
+
+## Impact
+
+- **Affected file**: `backend/app/services/pdf_generator_service.py`
+- **User experience**: OCR Track reflow PDF will contain all text content (matching Layout PDF)
+- **Translation**: Reflow translated PDF will also work correctly for OCR Track
+
+## Migration
+
+- No data migration required
+- Existing `raw_ocr_regions.json` files contain all necessary data
+- No API changes