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/.env.example

@@ -41,6 +41,25 @@ ADMIN_PASSWORD=ChangeMe123!
 # Path to Firebase service-account JSON (falls back to backend/firebase-service-account.json)
 FIREBASE_SERVICE_ACCOUNT_PATH=
 
+# --- Valkey availability mirror cadences ---
+#
+# All env-driven per backend/CLAUDE.md Config-Source Convention. Defaults match
+# requirement/valkey-online-mirror-plan.md. Floor-clamped by their getters.
+
+# How often the auto-offline sweep checks heartbeat freshness (seconds).
+# The staleness threshold itself (`stale_after_seconds`) is CC-tunable via app_config.
+MITRA_AUTO_OFFLINE_SWEEP_SECONDS=30
+
+# How often heartbeat timestamps are batched from Valkey → Postgres (seconds).
+# Per-ping heartbeat writes go to Valkey only; this preserves forensic
+# `last_heartbeat_at` in Postgres with up to <interval> seconds of lag.
+HEARTBEAT_MIRROR_INTERVAL_SECONDS=60
+
+# How often Valkey state is re-derived from Postgres to heal drift (seconds).
+# Belt-and-braces against failed best-effort Valkey writes, out-of-band Postgres
+# mutations, or evictions. Set to 0 to disable (not recommended).
+VALKEY_ONLINE_MIRROR_SWEEP_SECONDS=300
+
 # --- Phase 5: Xendit (dev-safe defaults: integration disabled) ---
 #
 # Flip XENDIT_ENABLED=true in staging/prod once secret + webhook token are populated.