Fix: Update all old-style imports inside packages to new paths

65 files in klausur-service packages + 3 in backend-lehrer packages
had stale imports referencing deleted shim modules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/messenger/conversations.py

@@ -266,7 +266,7 @@ async def send_message(conversation_id: str, message: MessageBase):
 
             if contact and contact.get("email"):
                 try:
-                    from email_service import email_service
+                    from services.email import email_service
 
                     result = email_service.send_messenger_notification(
                         to_email=contact["email"],