[ZK] ZK freelancer credentials (Sybil resistance)

[Gbangbolaoluwagbemiga]

User story

The marketplace needs Sybil resistance: prevent one person from spinning up many wallets to farm
reputation or spam applications — without forcing freelancers to dox their identity on-chain.

What needs to be done

• Choose a credential source (e.g. a privacy-preserving identity/KYC provider, proof-of-personhood,
  or verifiable skill credential) — evaluate in [ZK] Research spike: ZK verification on Soroban #35
• Issue a nullifier so each real human can register at most one verified marketplace identity,
  while the wallet ↔ identity link stays private
• Contract: store the nullifier set; reject re-use; expose is_verified(address)
• Frontend: "Verified human / credentialed" badge on freelancer profiles and applications

Acceptance criteria

• One person cannot verify two wallets (nullifier collision rejected)
• Wallet identity is not revealed on-chain
• Verified status surfaced in FreelancersPage and application views

Relevant files

contracts/secureflow/src/marketplace.rs, src/pages/FreelancersPage.tsx

Depends on #35.