feat(Explorer): Enhance metric extraction, source transparency, and UI display

- **Standardization & Formula Logic:** Fixed NameError/SyntaxError in formula parser; added support for comments and capitalized placeholders.
- **Source URL Tracking:** Extended DB schema and cascade logic to store and track specific source URLs.
- **Frontend & UI:**
  - Added 'Standardized Potential' display in Inspector.
  - Added clickable source link with icon.
  - Fixed Settings tab layout collapse (flex-shrink-0).
- **Export Capabilities:**
  - Single-company JSON export now includes full quantitative metadata.
  - New global CSV export endpoint /api/companies/export.
- **System Integrity:**
  - Fixed Notion sync typo ('Stanardization').
  - Corrected Nginx proxy routing and FastAPI route ordering.
  - Ensured DB persistence via explicit docker-compose volume mapping.

company-explorer/backend/services/scraping.py

@@ -291,4 +291,4 @@ def scrape_website_content(url: str) -> Optional[str]:
             return text
     except Exception as e:
         logger.error(f"Scraping error for {url}: {e}")
-    return None
+    return ""