[612] Implement node reputation scoring for decentralized RPC node selection

[Max-Owolabi]

Summary

• Implements NodeReputationService with 40/30/20/10 weighted scoring (success rate, inverse latency, freshness, liveness)
• Adds NodeSelector with weighted random selection and primary/secondary/tertiary fallback chains
• Persists scores in Redis with 5-minute TTL via NodeScoreCache
• Dead node detection (5 consecutive failures), circuit breaker when all nodes dead, and monitoring metrics

Test plan

• Unit tests for NodeReputationService, NodeSelector, NodeScoreCache, and NodeReputationMetrics
• Verified reputation formula, dead node detection, circuit breaker, and fallback chain ordering

Closes #612

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