[Backend] Optimize SQL queries in Database Pooler

[DeFiVC]

Closes #760

Changes

• Add LRU query result cache with TTL expiration (db-query-cache.js)
• Add composite indexes for frequently executed queries
• Integrate caching with the optimized query module
• Add Prometheus metrics for cache hit/miss tracking

Implementation

• backend/src/lib/db-query-cache.js — LRU cache (max 500 entries, 30s TTL)
• backend/src/lib/db-query-cache.test.js — Full test coverage
• backend/migrations/20260529000001_optimize_db_pooler_indexes.js — Composite indexes
• backend/src/lib/db-pooler-optimized.js — Integrated optimized query execution
• backend/src/lib/db-pooler-optimized.test.js — Tests for all pooler features

Test Output

All tests pass for query cache, LRU eviction, TTL expiration, and cache key generation.

Security Notes

• Cache keys use SHA-256 hashing (no user-controlled keys)
• Only SELECT queries cached (writes bypass cache)
• LRU eviction prevents memory exhaustion
• TTL prevents stale data serving

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