Phase 6: Valkey availability mirror — move read path off Postgres

Mitra-availability state (online flag, deactivated flag, per-mitra session
count, heartbeat liveness) mirrored into Valkey so the customer beacon
+ pairing blast + dashboard counts no longer hit Postgres on the hot path.
Postgres remains the durable source of truth; Valkey state is fully
derivable via seedFromPostgres on startup + reconnect.

Schema
- mitras:online           SET    — mirror of is_online
- mitras:deactivated      SET    — mirror of is_active=false
- mitra:capacity:<id>     STRING — active+pending_payment session count
- mitra💓<id>    STRING — ISO timestamp of last ping
- availability:snapshot   JSON   — beacon cache, TTL 10s, cluster-shared

Write paths (Postgres first, best-effort Valkey)
- setOnline/setOffline mirror SADD/SREM + heartbeat SET/DEL
- updateMitraStatus mirrors mitras:deactivated AND revokes auth_sessions
  on deactivate (bounds the "ghost online" window to access-token TTL)
- heartbeat is Valkey-only on the hot path; the per-ping Postgres UPDATE
  on last_heartbeat_at is eliminated (was 1,200 ops/min at prod scale)
- chat_session lifecycle (accept/end/reroute/extension/expiry) calls
  recomputeCapacityForMitra after each UPDATE — derive-from-truth avoids
  the bookkeeping risk of per-transition INCR/DECR

Read paths (Valkey-first, Postgres fallback on Valkey error)
- isMitraReachable: SISMEMBER mitras:online + heartbeat freshness
- findAvailableMitras: SDIFF + pipelined GETs, filter by capacity + heartbeat
- countAvailableMitrasFromCache: Valkey-driven, cached cluster-wide 10s TTL
- dashboard online count: SCARD
- Each reader wraps Valkey ops in try/catch → Postgres fallback on outage

Heartbeat path on /api/mitra/status/heartbeat
- resolveMitra preHandler replaced with heartbeatGuard: SISMEMBER on
  mitras:deactivated (~0 DB hits per ping). Falls back to full DB
  resolveMitra if Valkey is unreachable so a Valkey outage doesn't
  silently accept heartbeats from deactivated mitras.

Three sweeps, env-configurable cadences
- MITRA_AUTO_OFFLINE_SWEEP_SECONDS (30) — Valkey-driven stale detection
- HEARTBEAT_MIRROR_INTERVAL_SECONDS (60) — batched UPSERT writes
  Valkey timestamps to Postgres last_heartbeat_at via UNNEST (1 statement
  per cycle, idempotent across instances)
- VALKEY_ONLINE_MIRROR_SWEEP_SECONDS (300) — periodic reseed heals drift

Startup
- restoreActiveTimers → seedFromPostgres → bind listeners
- onValkeyReady re-runs the seed on every reconnect (cold start + reseed
  on Valkey restart, no manual intervention)

Failure semantics
- Read fallback: every Valkey read wrapped, falls back to existing
  Postgres JOIN query — system stays correct during Valkey outage,
  performance degrades not breaks
- Write best-effort: Postgres write commits before Valkey is touched;
  Valkey errors log + continue; reconciliation sweep heals drift
- Auto-offline sweep aborts entirely on Valkey error (does NOT mass-
  offline via Postgres scan during Valkey hiccup)

Tests
- New: 32 integration tests in mitra-status.valkey-mirror.test.js
  covering seed, write-through, fallbacks, capacity lifecycle,
  auto-offline sweep, heartbeat mirror, deactivation flow, beacon cache
- Updated: fixtures.js seeds Valkey alongside Postgres when isOnline=true
- Updated: helpers/db.js resetDb also flushes test Valkey
- Fixed 2 pre-existing session-timer flakes (string IDs failed uuid
  parse; vi.advanceTimersByTimeAsync raced real Postgres I/O)
- All 124/124 backend tests pass (was 90/92)

Docs
- requirement/valkey-online-mirror-plan.md — canonical plan
- requirement/valkey-online-mirror-testing.md — manual E2E checklist
- requirement/deployment.md — infra + Valkey persistence guidance for
  prod (Memorystore Standard tier recommended; migration from
  self-hosted Valkey is zero-downtime via reseed-from-Postgres)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/services/dashboard.service.js

@@ -1,19 +1,33 @@
 import { getDb } from '../db/client.js'
+import * as valkey from '../plugins/valkey.js'
+import { VK_MITRAS_ONLINE } from './mitra-status.service.js'
 import { SessionStatus, TopicSensitivity } from '../constants.js'
 
 const sql = getDb()
 
+// Valkey-fast SCARD with Postgres fallback. The CC dashboard polls every few
+// seconds; SCARD is sub-ms so this keeps the dashboard responsive at any scale.
+const getOnlineMitrasCount = async () => {
+  try {
+    return await valkey.scard(VK_MITRAS_ONLINE)
+  } catch (err) {
+    console.warn('[dashboard] valkey unavailable, falling back to DB:', err.message)
+    const [{ c }] = await sql`SELECT COUNT(*)::int AS c FROM mitra_online_status WHERE is_online = true`
+    return c
+  }
+}
+
 export const getDashboardStats = async () => {
   const [
     [{ active_chats }],
-    [{ online_mitras }],
+    online_mitras,
     [{ pending_requests }],
     [{ sensitive_total }],
     [{ sensitive_last_30d_total }],
     [{ sensitive_last_30d_sensitive }],
   ] = await Promise.all([
     sql`SELECT COUNT(*) AS active_chats FROM chat_sessions WHERE status IN (${SessionStatus.ACTIVE}, ${SessionStatus.PENDING_PAYMENT})`,
-    sql`SELECT COUNT(*) AS online_mitras FROM mitra_online_status WHERE is_online = true`,
+    getOnlineMitrasCount(),
     sql`SELECT COUNT(*) AS pending_requests FROM chat_sessions WHERE status IN (${SessionStatus.SEARCHING}, ${SessionStatus.PENDING_ACCEPTANCE})`,
     sql`SELECT COUNT(*) AS sensitive_total FROM chat_sessions WHERE topic_sensitivity = ${TopicSensitivity.SENSITIVE}`,
     sql`SELECT COUNT(*) AS sensitive_last_30d_total FROM chat_sessions WHERE created_at >= NOW() - INTERVAL '30 days'`,