Implement allowance expiration with ledger-based TTL enforcement

[p3ris0n]

Summary

The current approve function accepts an _exp parameter but ignores it. Implement full allowance expiration logic so approvals auto-expire after a specified ledger.

Requirements

• Change allowance storage from i128 to AllowanceEntry { amount: i128, expiration_ledger: u32 }
• Modify approve() to store expiration ledger
• Modify read_allowance() to check current ledger vs expiration
• Modify transfer_from() and burn_from() to enforce expiration
• Add increase_allowance and decrease_allowance
• Emit updated events
• Migrate tests and add expiry tests
• SDK updates

[paulinaapeh8]

@paulinaapeh8 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Would like to work on this issue

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[elijahbok14-sudo]

@elijahbok14-sudo has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

pls, can i work on this

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[ikemHood]

@ikemHood has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I can get this done asap

ETA: 1hrs
Please assign

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[acmenoch]

@acmenoch has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello i would like to work on this issue, please assign me

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[42natus]

@42natus has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can i work on this, please?

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