fix: disable chart recognition due to PaddlePaddle 3.0.0 API limitation

PaddleOCR-VL chart recognition model requires `fused_rms_norm_ext` API
which is not available in PaddlePaddle 3.0.0 stable release.

Changes:
- Set use_chart_recognition=False in PP-StructureV3 initialization
- Remove unsupported show_log parameter from PaddleOCR 3.x API calls
- Document known limitation in openspec proposal
- Add limitation documentation to README
- Update tasks.md with documentation task for known issues

Impact:
- Layout analysis still detects/extracts charts as images ✓
- Tables, formulas, and text recognition work normally ✓
- Deep chart understanding (type detection, data extraction) disabled ✗
- Chart to structured data conversion disabled ✗

Workaround: Charts saved as image files for manual review

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

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

openspec/changes/add-gpu-acceleration-support/proposal.md

@@ -49,3 +49,32 @@ PaddleOCR supports CUDA GPU acceleration which can significantly improve OCR pro
   - Fully backward compatible - existing CPU-only installations continue to work
   - No breaking changes to API or configuration
   - Existing installations can opt-in by re-running setup script on GPU-enabled hardware
+
+## Known Issues and Limitations
+
+### Chart Recognition Feature Disabled (PaddlePaddle 3.0.0 API Limitation)
+
+**Issue**: Chart recognition feature in PP-StructureV3 is currently disabled due to API incompatibility.
+
+**Root Cause**:
+- PaddleOCR-VL chart recognition model requires `paddle.incubate.nn.functional.fused_rms_norm_ext` API
+- PaddlePaddle 3.0.0 stable only provides `fused_rms_norm` (base version)
+- The extended version `fused_rms_norm_ext` is not yet available in stable release
+
+**Impact**:
+- ✅ **Still Works**: Layout analysis can detect and extract chart/figure regions as images
+- ✅ **Still Works**: Tables, formulas, and text recognition all function normally
+- ❌ **Disabled**: Deep chart understanding (chart type detection, data extraction, axis/legend parsing)
+- ❌ **Disabled**: Converting chart content to structured data (JSON, tables)
+
+**Workaround**:
+- Set `use_chart_recognition=False` in PP-StructureV3 initialization
+- Charts are saved as image files but content is not analyzed
+
+**Future Resolution**:
+- Wait for PaddlePaddle 3.0.x/3.1.x update that adds `fused_rms_norm_ext` API
+- Or use PaddlePaddle develop version (unstable, not recommended for production)
+
+**Code Location**: [backend/app/services/ocr_service.py:216](../../backend/app/services/ocr_service.py#L216)
+
+**Status**: Documented limitation, pending PaddlePaddle framework update