feat: contract verification #50

Allow anyone to verify a smart contract by submitting source code + compiler settings. The backend compiles and checks the bytecode matches what's deployed on-chain; on success, ABI and source are stored and surfaced in the UI.

Backend

• POST /api/contracts/:address/verify — accepts source files, compiler version, optimization settings; compiles via solc and validates bytecode match; upserts into contract_abis
• GET /api/contracts/:address — returns verification status, ABI, source

Frontend

• Verification form on the contract/address detail page (visible when unverified)
• Once verified: show source code, ABI viewer, and decoded function calls in transaction logs