(SP: 1) [Backend] Move session heartbeat from DB writes to Redis sorted set

[LesiaUKR]

Goal

Eliminate DB writes from the session heartbeat (/api/sessions/activity). Currently each call does 2-3 SQL operations (INSERT/UPDATE + COUNT + occasional DELETE). Move to Redis sorted set — 3 commands pipelined into 1 HTTP call, zero DB writes.

Scope

• Add heartbeatViaRedis(sessionId) — Redis sorted set online_sessions with pipeline (ZADD + ZREMRANGEBYSCORE + ZCARD)
• Extract existing DB logic into heartbeatViaDb(sessionId) as fallback
• POST handler: Redis first, DB fallback via nullish coalescing (??)
• SESSION_TIMEOUT_MS constant replacing 2 duplicate calculations
• Pattern: same as lib/quiz/quiz-answers-redis.ts — getRedisClient() null check + try/catch

Expected impact

• Zero DB writes in normal operation; active_sessions table untouched when Redis is available
• 2-3 SQL queries per navigation replaced by 1 Redis HTTP call

Out of scope

• Client-side throttle changes (Issue (SP: 2) [Page] Home -> Hero  #4)
• Removing active_sessions table (kept as fallback)

What was implemented

• heartbeatViaRedis(sessionId) — Redis sorted set online_sessions with pipeline (ZADD + ZREMRANGEBYSCORE + ZCARD = 1 HTTP call)
• heartbeatViaDb(sessionId) — existing DB logic extracted as fallback
• POST handler: Redis first, DB fallback via ?? (nullish coalescing)
• SESSION_TIMEOUT_MS constant replacing 2 duplicate calculations
• Zero DB writes in normal operation; active_sessions table untouched when Redis is available
• Verified in Upstash CLI: sorted set populating correctly, scores updating on navigation

Files changed:

• frontend/app/api/sessions/activity/route.ts — rewritten with Redis-first pattern