feat: add translated PDF export with layout preservation

Adds the ability to download translated documents as PDF files while
preserving the original document layout. Key changes:

- Add apply_translations() function to merge translation JSON with UnifiedDocument
- Add generate_translated_pdf() method to PDFGeneratorService
- Add POST /api/v2/translate/{task_id}/pdf endpoint
- Add downloadTranslatedPdf() method and PDF button in frontend
- Add comprehensive unit tests (52 tests: merge, PDF generation, API endpoints)
- Archive add-translated-pdf-export proposal

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

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

openspec/changes/archive/2025-12-02-add-translated-pdf-export/design.md

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+# Design: Add Translated PDF Export
+
+## Context
+
+The Tool_OCR project has implemented document translation using DIFY AI API, producing JSON files with translated content mapped by element_id. The existing PDF generator (`PDFGeneratorService`) can generate layout-preserving PDFs from UnifiedDocument but has no translation support.
+
+**Key Constraint**: The PDF generator uses element_id to position content. Translation JSON uses the same element_id mapping, making merging straightforward.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Generate PDF with translated text preserving original layout
+- Support all processing tracks (DIRECT, OCR, HYBRID)
+- Maintain backward compatibility with existing PDF export
+- Support table cell translation rendering
+
+**Non-Goals:**
+- Font optimization for target language scripts
+- Interactive editing of translations
+- Bilingual PDF output (original + translated side-by-side)
+
+## Decisions
+
+### Decision 1: Translation Merge Strategy
+
+**What**: Merge translation data into UnifiedDocument in-memory before PDF generation.
+
+**Why**: This approach:
+- Reuses existing PDF rendering logic unchanged
+- Keeps translation and PDF generation decoupled
+- Allows easy testing of merged document
+
+**Implementation**:
+```python
+def apply_translations(
+    unified_doc: UnifiedDocument,
+    translations: Dict[str, Any]
+) -> UnifiedDocument:
+    """Apply translations to UnifiedDocument, returning modified copy"""
+    doc_copy = unified_doc.copy(deep=True)
+    for page in doc_copy.pages:
+        for element in page.elements:
+            if element.element_id in translations:
+                translation = translations[element.element_id]
+                if isinstance(translation, str):
+                    element.content = translation
+                elif isinstance(translation, dict) and 'cells' in translation:
+                    # Handle table cells
+                    apply_table_translation(element, translation)
+    return doc_copy
+```
+
+**Alternatives considered**:
+- Modify PDF generator to accept translations directly - Would require significant refactoring
+- Generate overlay PDF with translations - Complex positioning logic
+
+### Decision 2: API Endpoint Design
+
+**What**: Add `POST /api/v2/translate/{task_id}/pdf?lang={target_lang}` endpoint.
+
+**Why**:
+- Consistent with existing `/translate/{task_id}` pattern
+- POST allows future expansion for PDF options
+- Clear separation from existing `/download/pdf` endpoint
+
+**Response**: Binary PDF file with `application/pdf` content-type.
+
+### Decision 3: Frontend Integration
+
+**What**: Add conditional "Download Translated PDF" button in TaskDetailPage.
+
+**Why**:
+- Only show when translation is complete
+- Use existing download pattern from PDF export
+
+## Risks / Trade-offs
+
+| Risk | Mitigation |
+|------|------------|
+| Large documents may timeout | Use existing async pattern, add progress tracking |
+| Font rendering for CJK scripts | Rely on existing NotoSansSC font registration |
+| Translation missing for some elements | Use original content as fallback |
+
+## Migration Plan
+
+No migration needed - additive feature only.
+
+## Open Questions
+
+1. Should we support downloading multiple translated PDFs in batch?
+2. Should translated PDF filename include source language as well as target?