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/database.py

@@ -51,6 +51,7 @@ class Company(Base):
     standardized_metric_unit = Column(String, nullable=True) # e.g., "m²"
     metric_source = Column(String, nullable=True)            # "website", "wikipedia", "serpapi"
     metric_proof_text = Column(Text, nullable=True)          # Snippet showing the value (e.g. "2,0 Mio Besucher (2020)")
+    metric_source_url = Column(Text, nullable=True)          # URL where the proof was found
     metric_confidence = Column(Float, nullable=True)         # 0.0 - 1.0
     metric_confidence_reason = Column(Text, nullable=True)   # Why is it high/low?