Verify on-chain USDC transfer details before granting paid access

[AnkanMisra]

Summary

The core technical step for real settlement support is verifying that a qualifying USDC transfer actually happened on-chain before granting paid access.

Current State

The current verifier and gateway path validate signed authorization data only. They do not validate transaction receipt state, token contract, recipient, amount, or confirmation status for a real on-chain USDC transfer.

What to build

Implement the on-chain verification path needed to confirm a qualifying USDC transfer before granting access in an on-chain settlement mode.

Acceptance Criteria

• On-chain verification checks transaction receipt success.
• Verification checks expected token contract, recipient, amount, and chain ID.
• Confirmation requirements are explicit and documented.
• Missing receipt, wrong token, wrong recipient, wrong amount, wrong chain, and RPC failure cases return safe public errors.
• Tests cover both valid settlement and the main negative cases.

Non-goals

• Do not define payment-mode configuration in this issue.
• Do not solve transaction replay/deduplication in this issue.
• Do not change the signature-only demo path beyond what is required for clean mode separation.

Verification

• Run the relevant gateway/verifier test suites.
• Exercise a valid and invalid settlement path against the chosen RPC/test setup.

Blocked by

Parent umbrella: #161

[Stewartsson]

@AnkanMisra i am a GSSoC contributor can you please assign this to me o i can work on it