chore: update deployments and clean repository

Updated deployment addresses:
- base-mainnet.json: new V2 deployment addresses from Base mainnet
  - NFT: 0xab5B5Be29048339De2Bf79c51c1634adC987deFb
  - Payment: 0x9a4F17a4dE62be11738d15b39bb0Dfba88cA9B74
  - PaymentLink: 0xDe9aD4eD1909204319AF94605d40CA5886fB97f8
- base-sepolia.json: new V2 deployment addresses from Base Sepolia
  - NFT: 0x8C49fb4c7512A238D7A6fC9B612A0deFFA4890f5
  - Payment: 0x73CBc9E6Ac0b17Ba0b42c9c68Fbb3B9f55485540
  - PaymentLink: 0x5C0D87f9a97eF5592c47D7649668b1a2F9a03DcA

Updated README.md:
- Updated contract addresses to latest deployment
- Changed InvoicePayment and PaymentLink to show UUPS upgradeable status
- Fixed OpenZeppelin version from v5.0.2 to v4.9.6 (correct version)
- Added "Proxy" suffix to contract names for clarity

Updated docs/INVOICE_STATE_MACHINE.md:
- Updated contract addresses from TBD to actual deployed addresses

Removed old/obsolete files:
- .gas-snapshot (outdated gas snapshot)
- foundry.lock (not needed in git)
- INVOICE_STATE_MACHINE.md (duplicate in root, kept in docs/)
- deployments/base-mainnet-v2.json (old deployment)
- deployments/base-sepolia-v2.json (old deployment)

All contracts verified on BaseScan.

Author: diorwave