SSE endpoint for paid invoices.

also move very essential stuff from core/tasks.py to tasks.py so things
are more organized.

lnbits/db.py

@@ -12,6 +12,9 @@ class Database:
         self.cursor = self.connection.cursor()
         self.closed = False
 
+    def close(self):
+        self.__exit__(None, None, None)
+
     def __enter__(self):
         return self