Fix unbounded receipt minting by adding MAX_USER_RECEIPTS cap

[Peolite001]

Summary

This PR bounds the number of receipts a single account can mint by introducing a MAX_USER_RECEIPTS cap. This prevents individual accounts from bloating the contract's storage footprint and slowing down the TTL sweep.

Type of change

• Bug fix
• New feature
• Documentation update
• Refactor / chore
• Smart contract change

Related issue

Closes #884

Changes

• Defined a MAX_USER_RECEIPTS constant set to 1,000 in contracts/finchippay-contract/src/lib.rs.
• Updated the mint_receipt function to enforce the cap and panic with "User receipt limit reached" if the limit is exceeded.
• Added documentation for the new receipt cap in contracts/finchippay-contract/README.md under the "Bounded inputs" section.
• Added boundary tests (test_mint_receipt_cap and test_mint_receipt_cap_exceeded) to verify that minting works up to the limit and cleanly reverts when the cap is crossed.

Testing

• Tested locally on Testnet
• Added/updated unit tests
• Manually tested UI flow

Screenshots (if UI change)

N/A

Checklist

• My code follows the project style
• I've updated docs if needed
• No console errors or warnings
• I've rebased on latest main

[github-actions]

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[Topmatrixmor2014]

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