fix: handle None image_path safely to prevent AttributeError

Fix bug introduced in previous commit where image_path=None caused
AttributeError when calling .lower() on None value.

**Problem**:
Setting image_path to None for table placeholders caused crashes at:
- Line 415: 'table' in img.get('image_path', '').lower()
- Line 453: 'table' not in img.get('image_path', '').lower()

When key exists but value is None, .get('image_path', '') returns None
(not default value), causing .lower() to fail.

**Solution**:
Use img.get('type') == 'table' to identify table entries instead of
checking image_path string. This is:
- More explicit and reliable
- Safer (no string operations on potentially None values)
- Cleaner code

**Changes**:
- Line 415: Check img.get('type') == 'table' for table count
- Line 453: Filter using img.get('type') != 'table' and image_path is not None
- Added informative log message showing table count

**Verification**:
draw_image_region already safely handles None/empty image_path (lines 1013-1015)
by returning early if not image_path_str.

Task 2.1 now fully functional without crashes.

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

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

backend/app/services/pdf_generator_service.py

@@ -412,9 +412,9 @@ class PDFGeneratorService:
 
             # Filter text regions to avoid overlap with tables/images
             regions_to_avoid = images_metadata
-            table_count = len([img for img in images_metadata if 'table' in img.get('image_path', '').lower()])
+            table_count = len([img for img in images_metadata if img.get('type') == 'table'])
 
-            logger.info(f"過濾文字區域: {len(regions_to_avoid)} 個區域需要避免")
+            logger.info(f"過濾文字區域: {len(regions_to_avoid)} 個區域需要避免 (含 {table_count} 個表格)")
 
             filtered_text_regions = self._filter_text_in_regions(text_regions, regions_to_avoid)
 
@@ -450,7 +450,8 @@ class PDFGeneratorService:
                 page_image_regions = [
                     img for img in images_metadata
                     if img.get('page') == page_num - 1
-                    and 'table' not in img.get('image_path', '').lower()
+                    and img.get('type') != 'table'
+                    and img.get('image_path') is not None  # Skip table placeholders
                 ]
 
                 # Draw in layers: images → tables → text