fix: improve concurrency, configurability, and safety

[IchenDEV]

Summary

P0-1: Round counter concurrency fix

• Use atomic UPDATE instead of read-then-write to avoid race conditions

P1-1: RRF k parameter configurable

• Add SearchConfig interface with rrfK, bm25Weight, vecWeight options
• mergeRRF() now accepts optional config parameter

P1-2: Graph traversal hardening

• Hard limit max depth to 3 (prevent deep traversal attacks)
• Hard limit max nodes to 1000 (prevent memory exhaustion)

P2: Retry utility

• Add withRetry() utility with exponential backoff
• Supports custom max attempts, backoff delay, and retry callback

Build fixes

• Add @types/node to dependencies
• Fix TypeScript compilation errors