not a teacher
"Do you think you could lend me some money until next week?"
It means: Do you think you can give me some money on the understanding that I will pay you back next week?
Your definition (1) is closest.
"Could you lend me some money by next week?" means: Could you give me some money by the time next week comes, on the understanding that I will pay you back later (at an as yet unspecified time)?"