feat: Implement unsubscribe link for marketing automation [31188f42]

This commit introduces a new unsubscribe feature to allow contacts to opt-out
from marketing automation.

Key changes include:
- Database schema migration: Added  (UUID) to the  model.
- Data population: Implemented a script to assign unique tokens to existing contacts.
- API endpoint: Created a public GET  endpoint to handle opt-out requests.
- Automation: New contacts automatically receive an unsubscribe token upon creation.
- Integration: The full unsubscribe link is now returned via the provisioning API
  for storage in SuperOffice UDFs (ProgID: SuperOffice:9).
- Documentation: Updated  and
  to reflect the new feature and its integration requirements.
- Added  for quick overview and next steps.

company-explorer/backend/database.py

@@ -107,6 +107,9 @@ class Contact(Base):
     role = Column(String) # Operativer Entscheider, etc.
     status = Column(String, default="") # Marketing Status
     
+    # New field for unsubscribe functionality
+    unsubscribe_token = Column(String, unique=True, index=True, nullable=True)
+
     is_primary = Column(Boolean, default=False)
     
     created_at = Column(DateTime, default=datetime.utcnow)