feat: implement hybrid image extraction and memory management

Backend:
- Add hybrid image extraction for Direct track (inline image blocks)
- Add render_inline_image_regions() fallback when OCR doesn't find images
- Add check_document_for_missing_images() for detecting missing images
- Add memory management system (MemoryGuard, ModelManager, ServicePool)
- Update pdf_generator_service to handle HYBRID processing track
- Add ElementType.LOGO for logo extraction

Frontend:
- Fix PDF viewer re-rendering issues with memoization
- Add TaskNotFound component and useTaskValidation hook
- Disable StrictMode due to react-pdf incompatibility
- Fix task detail and results page loading states

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

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

backend/app/services/pp_structure_enhanced.py

@@ -21,6 +21,8 @@ except ImportError:
 import paddle
 from paddleocr import PPStructureV3
 from app.models.unified_document import ElementType
+from app.core.config import settings
+from app.services.memory_manager import prediction_context
 
 logger = logging.getLogger(__name__)
 
@@ -96,8 +98,22 @@ class PPStructureEnhanced:
         try:
             logger.info(f"Enhanced PP-StructureV3 analysis on {image_path.name}")
 
-            # Perform structure analysis
-            results = self.structure_engine.predict(str(image_path))
+            # Perform structure analysis with semaphore control
+            # This prevents OOM errors from multiple simultaneous predictions
+            with prediction_context(timeout=settings.service_acquire_timeout_seconds) as acquired:
+                if not acquired:
+                    logger.error("Failed to acquire prediction slot (timeout), returning empty result")
+                    return {
+                        'has_parsing_res_list': False,
+                        'elements': [],
+                        'total_elements': 0,
+                        'images': [],
+                        'tables': [],
+                        'element_types': {},
+                        'error': 'Prediction slot timeout'
+                    }
+
+                results = self.structure_engine.predict(str(image_path))
 
             all_elements = []
             all_images = []