docs: add backlog 999.13 — reduce event write-back latency (outbox 15s drain → event-driven)

.planning/ROADMAP.md

@@ -441,3 +441,22 @@ Plans:
 Plans:
 
 - [ ] TBD (promote with /gsd-review-backlog when ready)
+
+### Phase 999.13: Reduce event write-back latency to the calendar provider (outbox drain) (BACKLOG)
+
+**Goal:** [Captured for future planning] Calendar create/edit/delete writes are enqueue-only (`calendarOutbox`, 202 optimistic-accept; D-12/D-05 — no Fastmail call in the route) and flushed to Fastmail by `runOutboxDrain` on a **15-second `setInterval`** (`apps/api/src/broker/outboxWorker.ts`). So a change can take up to ~15s to land in Fastmail (and longer to reflect back in the app, which depends on the separate 5-min poller). Reduce that perceived sync delay so edits feel near-immediate.
+
+**Options to weigh when picking this up:**
+- **Event-driven drain (preferred):** trigger an outbox drain immediately after a successful enqueue (in-process signal, or Redis pub/sub which is already available) so the write fires within ~1s instead of waiting for the next tick — keep the 15s `setInterval` as a fallback/retry sweep. Must preserve the existing per-row etag/412 handling and the rapid-successive-edit ordering (see outboxWorker comments ~L312 — each edit carries its enqueue-time etag).
+- **Shorter interval:** simplest, but more idle DB polling; a floor (e.g. 3–5s) trades latency for load.
+- **Faster read-back too:** the user also sees latency from the 5-min poller reflecting the change back. Consider invalidating/short-poll after a local write, or optimistic UI already covering it — confirm whether the perceived delay is the write (15s) or the read-back (5min).
+
+**Boundary:** the optimistic 202 + outbox durability design (create-before-delete, drain concurrency guard, fresh-etag-before-PUT) must be preserved — this is a latency tune, not a rewrite of the write path.
+
+**Context:** Surfaced 2026-06-10. Tags: calendar, write-back, outbox, latency, redis, performance.
+**Requirements:** TBD
+**Plans:** 0 plans
+
+Plans:
+
+- [ ] TBD (promote with /gsd-review-backlog when ready)