fix: add PaddlePaddle 2.x/3.x API compatibility layer

PaddlePaddle 3.0.0b2 has "Illegal instruction" error on current CPU.
Downgrade to stable 2.6.2 which works but uses different API.

Changes:
- Auto-detect PaddlePaddle version at runtime
- Use 'device' parameter for 3.x (device="gpu:0" or "cpu")
- Use 'use_gpu' + 'gpu_mem' parameters for 2.x
- Apply to both get_ocr_engine() and get_structure_engine()
- Log PaddlePaddle version in initialization messages

Current setup:
- paddlepaddle-gpu==2.6.2 (stable, CUDA compiled)
- paddleocr==3.3.1
- paddlex==3.3.9

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

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

backend/app/services/ocr_service.py

@@ -170,25 +170,48 @@ class OCRService:
             logger.info(f"Initializing PaddleOCR engine for language: {lang} (GPU: {self.use_gpu})")
 
             try:
-                # PaddleOCR 3.x uses 'device' parameter instead of 'use_gpu' and 'gpu_mem'
+                # Check PaddlePaddle version to use correct API
+                paddle_version = paddle.__version__
+                is_paddle_3x = paddle_version.startswith('3.')
+
+                if is_paddle_3x:
+                    # PaddlePaddle 3.x uses 'device' parameter
                     device = f"gpu:{settings.gpu_device_id}" if self.use_gpu else "cpu"
                     self.ocr_engines[lang] = PaddleOCR(
                         use_angle_cls=True,
                         lang=lang,
                         device=device,
                     )
-                logger.info(f"PaddleOCR engine ready for {lang} ({'GPU' if self.use_gpu else 'CPU'} mode)")
+                else:
+                    # PaddlePaddle 2.x uses 'use_gpu' and 'gpu_mem' parameters
+                    self.ocr_engines[lang] = PaddleOCR(
+                        use_angle_cls=True,
+                        lang=lang,
+                        use_gpu=self.use_gpu,
+                        gpu_mem=int(settings.gpu_memory_fraction * 1000) if self.use_gpu else 500,
+                    )
+                logger.info(f"PaddleOCR engine ready for {lang} (PaddlePaddle {paddle_version}, {'GPU' if self.use_gpu else 'CPU'} mode)")
 
             except Exception as e:
                 # If GPU initialization fails, fall back to CPU
                 if self.use_gpu:
                     logger.warning(f"GPU initialization failed, falling back to CPU: {e}")
                     self.use_gpu = False
+                    paddle_version = paddle.__version__
+                    is_paddle_3x = paddle_version.startswith('3.')
+
+                    if is_paddle_3x:
                         self.ocr_engines[lang] = PaddleOCR(
                             use_angle_cls=True,
                             lang=lang,
                             device="cpu",
                         )
+                    else:
+                        self.ocr_engines[lang] = PaddleOCR(
+                            use_angle_cls=True,
+                            lang=lang,
+                            use_gpu=False,
+                        )
                     logger.info(f"PaddleOCR engine ready for {lang} (CPU mode - fallback)")
                 else:
                     raise
@@ -206,7 +229,12 @@ class OCRService:
             logger.info(f"Initializing PP-StructureV3 engine (GPU: {self.use_gpu})")
 
             try:
-                # PaddleOCR 3.x uses 'device' parameter instead of 'use_gpu' and 'gpu_mem'
+                # Check PaddlePaddle version to use correct API
+                paddle_version = paddle.__version__
+                is_paddle_3x = paddle_version.startswith('3.')
+
+                if is_paddle_3x:
+                    # PaddlePaddle 3.x uses 'device' parameter
                     device = f"gpu:{settings.gpu_device_id}" if self.use_gpu else "cpu"
                     self.structure_engine = PPStructureV3(
                         use_doc_orientation_classify=False,
@@ -217,12 +245,28 @@ class OCRService:
                         layout_threshold=0.5,
                         device=device,
                     )
-                logger.info(f"PP-StructureV3 engine ready ({'GPU' if self.use_gpu else 'CPU'} mode)")
+                else:
+                    # PaddlePaddle 2.x uses 'use_gpu' and 'gpu_mem' parameters
+                    self.structure_engine = PPStructureV3(
+                        use_doc_orientation_classify=False,
+                        use_doc_unwarping=False,
+                        use_textline_orientation=False,
+                        use_table_recognition=True,
+                        use_formula_recognition=True,
+                        layout_threshold=0.5,
+                        use_gpu=self.use_gpu,
+                        gpu_mem=int(settings.gpu_memory_fraction * 1000) if self.use_gpu else 500,
+                    )
+                logger.info(f"PP-StructureV3 engine ready (PaddlePaddle {paddle_version}, {'GPU' if self.use_gpu else 'CPU'} mode)")
 
             except Exception as e:
                 # If GPU initialization fails, fall back to CPU
                 if self.use_gpu:
                     logger.warning(f"GPU initialization failed for PP-Structure, falling back to CPU: {e}")
+                    paddle_version = paddle.__version__
+                    is_paddle_3x = paddle_version.startswith('3.')
+
+                    if is_paddle_3x:
                         self.structure_engine = PPStructureV3(
                             use_doc_orientation_classify=False,
                             use_doc_unwarping=False,
@@ -232,6 +276,16 @@ class OCRService:
                             layout_threshold=0.5,
                             device="cpu",
                         )
+                    else:
+                        self.structure_engine = PPStructureV3(
+                            use_doc_orientation_classify=False,
+                            use_doc_unwarping=False,
+                            use_textline_orientation=False,
+                            use_table_recognition=True,
+                            use_formula_recognition=True,
+                            layout_threshold=0.5,
+                            use_gpu=False,
+                        )
                     logger.info("PP-StructureV3 engine ready (CPU mode - fallback)")
                 else:
                     raise