fix: properly complete task 2.1 - remove fake table image dependency

Correctly implement task 2.1 by completely removing dependency on fake
table_*.png references as originally intended.

**Changes**:
- Set table image_path to None instead of fake "table_*.png"
- Removed backward compatibility fallback that looked for fake table images
- Tables now exclusively use element's own bbox for rendering
- Kept bbox in images_metadata only for text overlap filtering

**Rationale**:
The previous implementation kept creating fake table_*.png references
and included fallback logic to find them. This defeated the purpose of
task 2.1 which was to eliminate dependency on non-existent image files.

Now tables render purely based on their own bbox data without any
reference to fake image files.

**Files Modified**:
- backend/app/services/pdf_generator_service.py:251-259 (fake path removed)
- backend/app/services/pdf_generator_service.py:874-891 (fallback removed)
- openspec/changes/pdf-layout-restoration/tasks.md (accurate status)

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

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

openspec/changes/pdf-layout-restoration/tasks.md

@@ -21,8 +21,8 @@
 
 ### 2. Fix Table Rendering
 - [x] 2.1 Remove dependency on fake image references
-  - [x] 2.1.1 Stop creating fake table_*.png references (kept for backward compatibility)
-  - [x] 2.1.2 Remove image lookup in draw_table_region (now uses direct bbox first)
+  - [x] 2.1.1 Stop creating fake table_*.png references (changed to None)
+  - [x] 2.1.2 Remove image lookup fallback in draw_table_region
 - [x] 2.2 Use direct bbox from table element
   - [x] 2.2.1 Get bbox from table_element.get("bbox")
   - [x] 2.2.2 Fallback to bbox_polygon if needed