fix(backend): bump cross-chain finalization wait to 2 minutes

[gianalarcon]

What

Raises CROSS_CHAIN_FINALIZATION_WAIT from 40s → 120s (2 min), applied uniformly to all chains (Base, Horizen, Arbitrum Sepolia).

Why

The zkVerify team confirmed proof aggregation now completes within ~2 minutes across supported networks, including Arbitrum Sepolia (previously slower). The executor waits this duration after a recent aggregation before polling for the cross-chain receipt; 40s was too short for the aggregation-to-finalization window, risking premature/stuck executions. 2 minutes matches the observed aggregation time.

"both issues should have been addressed: arbitrum sepolia aggregation time is down to 2 minutes, and support for Arbitrum One Mainnet has been finalized" — zkVerify team

Changes

• packages/backend/src/common/constants/timing.ts — CROSS_CHAIN_FINALIZATION_WAIT 40_000 → 120_000.
• packages/backend/src/transaction/transaction-executor.service.ts — log message now derives the duration from the constant (${CROSS_CHAIN_FINALIZATION_WAIT / 1000}s) instead of a hardcoded "40s", so it can't drift.

Notes

• This is a single global constant — there's no per-chain wait today, so Base/Horizen also move from 40s to 2 min. Per the decision to keep one uniform value.
• Overlaps with the same constant bump inside PR feat(arbitrum): add Arbitrum Sepolia as a destination chain #282 (Arbitrum support); landing it standalone here decouples the timing fix from that larger PR. If feat(arbitrum): add Arbitrum Sepolia as a destination chain #282 merges first this becomes a no-op; if this merges first, feat(arbitrum): add Arbitrum Sepolia as a destination chain #282's diff for this line drops out.

Validation

• No node_modules in this checkout, so local tsc/build not run. Change is a constant value + a log-string interpolation; CI yarn build (lint.yaml) will compile-check.

🤖 Generated with Claude Code

[gianalarcon]

Closing — this change should land via the Arbitrum work on feat/arbitrum-support (#282), which already sets CROSS_CHAIN_FINALIZATION_WAIT to 2 minutes. Consolidating there instead of a standalone PR.