feat: add bounty deadline refunds

[meteop-x]

Closes #3.

What changed:

• Store bounty token, amount, and deadline together in persistent bounty records.
• Reject bounty creation when the amount is non-positive or the deadline has already passed.
• Allow maintainer cancellation/refund only after the bounty deadline, then remove the stored bounty record.
• Replace the placeholder contract test with Soroban token tests covering lock, late refund, past-deadline create rejection, and early-cancel rejection.

Validation:

• cargo fmt --all -- --check
• cargo test