GET /api/rewards/{delegator}/{validator}
→ Returns cumulative earned rewards (claimed + unclaimed)

Example:
Val A + Del X earns 20 IP -> API returns 20 IP
Delegation ends, new one for same combination earns 10 IP -> API returns 30 IP

Current APIs/blocks don't provide this:
Pending rewards only (no history)
EL withdrawal blocks (no validator info)

This is a fundamental information for tracking rewards in multi-validator staking systems.