fix: retune NPC affinity gate

[JOY (JOY)]

Summary

• lower the frequent-interaction affinity gate so positive repeated NPC dialogue can continue building relationships
• add regression coverage for repeated accepted NPC speech and relationship growth

Local review

• Local code-review fallback: approved, no blockers found in the scoped tuning diff
• Boundary check: this only changes social-memory tuning; hostility and distance gates remain strict

Verification

• npm run build (backend/nakama)
• npm test (backend/nakama)
• git diff --check
• docker restart second-spawn-nakama and /v2/rpc/secondspawn_health smoke