feat: upgrade PP-StructureV3 models to latest versions

- Layout: PP-DocLayout-S → PP-DocLayout_plus-L (83.2% mAP)
- Table: Single model → Dual SLANeXt (wired/wireless)
- Formula: PP-FormulaNet_plus-L for enhanced recognition
- Add preprocessing flags support (orientation, unwarping)
- Update frontend i18n descriptions

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

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

backend/tests/services/test_layout_model.py

@@ -40,8 +40,8 @@ class TestLayoutModelMapping:
         assert 'cdla' in LAYOUT_MODEL_MAPPING
 
     def test_chinese_model_maps_to_pp_doclayout(self):
-        """Verify 'chinese' maps to PP-DocLayout-S"""
-        assert LAYOUT_MODEL_MAPPING['chinese'] == 'PP-DocLayout-S'
+        """Verify 'chinese' maps to PP-DocLayout_plus-L"""
+        assert LAYOUT_MODEL_MAPPING['chinese'] == 'PP-DocLayout_plus-L'
 
     def test_default_model_maps_to_publaynet_sentinel(self):
         """Verify 'default' maps to sentinel value for PubLayNet default"""
@@ -57,7 +57,7 @@ class TestLayoutModelEngine:
     """Test engine creation with different layout models"""
 
     def test_chinese_model_creates_engine_with_pp_doclayout(self):
-        """Verify 'chinese' layout model uses PP-DocLayout-S"""
+        """Verify 'chinese' layout model uses PP-DocLayout_plus-L"""
         ocr_service = OCRService()
 
         with patch.object(ocr_service, 'structure_engine', None):
@@ -70,7 +70,7 @@ class TestLayoutModelEngine:
                 mock_ppstructure.assert_called_once()
                 call_kwargs = mock_ppstructure.call_args[1]
 
-                assert call_kwargs.get('layout_detection_model_name') == 'PP-DocLayout-S'
+                assert call_kwargs.get('layout_detection_model_name') == 'PP-DocLayout_plus-L'
 
     def test_default_model_creates_engine_without_model_name(self):
         """Verify 'default' layout model does not specify model name (uses default)"""
@@ -121,7 +121,7 @@ class TestLayoutModelEngine:
                 call_kwargs = mock_ppstructure.call_args[1]
 
                 # Should use 'chinese' model as default
-                assert call_kwargs.get('layout_detection_model_name') == 'PP-DocLayout-S'
+                assert call_kwargs.get('layout_detection_model_name') == 'PP-DocLayout_plus-L'
 
 
 class TestLayoutModelCaching: