fix: stop fabricating fake EVM transaction hashes in broadcastToChains

[Xhaka00]

Overview

This PR fixes the misleading behavior where POST /v1/chains/broadcast/:projectId reported fabricated EVM transaction hashes. Now the route returns success: false with a clear "EVM submission not implemented" error for EVM chains (Ethereum/Polygon) until real on-chain submission via ethers/viem is implemented. Stellar submissions remain unchanged and still return real hashes from updateImpactScore().

Related Issue

Closes #

Changes

🚫 Stop Fabricating EVM Transaction Hashes

• [MODIFY] src/routes/chains.ts
  • Added an explicit EVM-chain guard in POST /v1/chains/broadcast/:projectId so Ethereum/Polygon requests no longer reach the hash-fabricating path in submitToChain().
  • EVM chains now return { chain, success: false, error: "EVM submission not implemented" } instead of a fake 0x... transaction hash.
  • Removed the txHash field from EVM responses — no fabricated hash is ever reported for a submission that never touched the network.
  • Stellar path is untouched and continues to return the real hash from updateImpactScore().

Verification Results

npm test -- src/routes/__tests__/chains.test.ts
✅ 6/6 passed

Live API acceptance check:
✅ POST /v1/chains/broadcast/:projectId (Ethereum) → { chain: "ethereum", success: false, error: "EVM submission not implemented" }
✅ POST /v1/chains/broadcast/:projectId (Polygon) → { chain: "polygon", success: false, error: "EVM submission not implemented" }
✅ POST /v1/chains/broadcast/:projectId (Stellar) → real hash from updateImpactScore(), success: true
✅ No fabricated 0x... hashes appear in any EVM response

Acceptance Criteria	Status
EVM chains never report a fabricated transaction hash	✅ success: false returned for Ethereum/Polygon; no txHash included
API fails clearly for unimplemented EVM submission	✅ Clear "EVM submission not implemented" error message returned
Stellar submission behavior is preserved	✅ Stellar still returns real hash from updateImpactScore() with success: true
No misleading success: true when nothing was submitted on-chain	✅ EVM results are success: false until real ethers/viem submission is implemented

Closes #504

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