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>

README.md

@@ -158,6 +158,24 @@ The system automatically detects and utilizes NVIDIA GPU hardware when available
 
 Check GPU status at: http://localhost:8000/health
 
+### Known Limitations
+
+**Chart Recognition (PP-StructureV3)**
+
+Due to API incompatibility between PaddleOCR 3.x and PaddlePaddle 3.0.0 stable, the chart recognition feature is currently disabled:
+
+- ✅ **Works**: Layout analysis detects and extracts charts/figures as image files
+- ✅ **Works**: Tables, formulas, and text recognition function normally
+- ❌ **Disabled**: Deep chart content understanding (chart type, data extraction, axis/legend parsing)
+- ❌ **Disabled**: Converting chart content to structured data
+
+**Technical Details**:
+- The PaddleOCR-VL chart recognition model requires `paddle.incubate.nn.functional.fused_rms_norm_ext` API
+- PaddlePaddle 3.0.0 stable only provides the base `fused_rms_norm` function
+- This limitation will be resolved when PaddlePaddle releases an update with the extended API
+
+**Workaround**: Charts are saved as images and can be viewed manually. For chart data extraction, consider using specialized chart recognition tools separately.
+
 ## API Endpoints
 
 ### Authentication