feat: convex claim

[jpsains]

Note

Medium Risk
Touches core heuristic classification (contract/topic matching), which can change categorization outcomes for other transactions if priorities or matches conflict, though the change is narrow and covered by fixtures/snapshots.

Overview
Adds support for categorizing Convex reward-claim transactions as CONVEX_CLAIM by recognizing the Convex staked CVX reward pool contract and the RewardPaid log topic as a CLAIM signal.

Updates the ethereum test suite to include a new CONVEX_CLAIM case (mock tx hash, expected v5/v6 readable labels, and snapshot) and adds a matching nock fixture for the transaction API response.

^{Reviewed by Cursor Bugbot for commit 8170913. Bugbot is set up for automated code reviews on this repo. Configure here.}