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/extension.service.js

@@ -1,7 +1,7 @@
 import { getDb } from '../db/client.js'
 import { sendToSessionParticipant, isUserOnlineWs } from '../plugins/websocket.js'
 import { extendSessionTimer, clearClosureGraceTimer, startClosureGraceTimer } from './session-timer.service.js'
-import { isMitraReachable } from './mitra-status.service.js'
+import { isMitraReachable, recomputeCapacityBySession } from './mitra-status.service.js'
 import { consumePaymentSession, failPaymentSession, getPaymentSession } from './payment.service.js'
 import { sendPushNotification } from './notification.service.js'
 import {
@@ -104,6 +104,7 @@ export const requestExtension = async (sessionId, customerId, { duration_minutes
 
   // Pause the session
   await sql`UPDATE chat_sessions SET status = ${SessionStatus.EXTENDING} WHERE id = ${sessionId}`
+  await recomputeCapacityBySession(sessionId)
 
   // Resolve timeout once so we can both surface it in the WS payload and start the server-side timer.
   const timeoutMs = await getExtensionTimeoutMs()
@@ -213,6 +214,7 @@ const finalizeExtension = async (extensionId, sessionId, accepted, viaTimeout) =
 
     // Resume session
     await sql`UPDATE chat_sessions SET status = ${SessionStatus.ACTIVE} WHERE id = ${extension.session_id}`
+    await recomputeCapacityBySession(extension.session_id)
 
     // Record transaction
     await sql`
@@ -249,6 +251,7 @@ const finalizeExtension = async (extensionId, sessionId, accepted, viaTimeout) =
     }
 
     await sql`UPDATE chat_sessions SET status = ${SessionStatus.CLOSING} WHERE id = ${extension.session_id}`
+    await recomputeCapacityBySession(extension.session_id)
 
     sendToSessionParticipant(sessionId, UserType.CUSTOMER, {
       type: WsMessage.EXTENSION_RESPONSE,
@@ -331,6 +334,7 @@ const timeoutExtension = async (extensionId, sessionId, mitraId) => {
 
   // Move session to closing & notify both parties (matches the explicit-reject UX).
   await sql`UPDATE chat_sessions SET status = ${SessionStatus.CLOSING} WHERE id = ${sessionId}`
+  await recomputeCapacityBySession(sessionId)
   sendToSessionParticipant(sessionId, UserType.CUSTOMER, {
     type: WsMessage.EXTENSION_RESPONSE,
     accepted: false,