docs: add planning/deferred-work.md register (records the OutboxRouter gap)

planning/README.md

@@ -65,3 +65,6 @@ _None._
   *shipped* state, not pending work.
 - **[lint-suppressions.md](lint-suppressions.md)** — audit of `noqa` /
   `ty: ignore` directives and why each one stays.
+- **[deferred-work.md](deferred-work.md)** — real-but-unscheduled items
+  raised in reviews/audits, parked with a revisit trigger (e.g. the
+  `OutboxRouter` DLQ/recorder forwarding gap).

planning/deferred-work.md

@@ -0,0 +1,31 @@
+# Deferred Work
+
+Items raised in reviews or audits that are real but not actionable now.
+Each is parked here with the reason it's deferred and the concrete trigger
+that should bring it back. This is the long-tail register — not a backlog
+of planned work. When an item is picked up it graduates to a spec/plan
+pair in [`active/`](active/); see [CLAUDE.md](../CLAUDE.md#workflow).
+
+As of the 2026-06-12 code + docs audit closure (PRs #61–#74), the audit
+backlog is empty. The items below are the remainder: technically real,
+but deliberately unscheduled pending a trigger.
+
+## Open
+
+### FastAPI integration
+
+- **`OutboxRouter` doesn't forward `dlq_table` / `metrics_recorder` /
+  `routers`.** These three `OutboxBroker.__init__` arguments are not
+  exposed on `OutboxRouter.__init__`, and the router constructs the broker
+  internally with no handle to inject a pre-built one — so a FastAPI user
+  **cannot** enable the dead-letter queue or the metrics-recorder seam
+  through the router at all. The only path today is a standalone
+  `OutboxBroker`. This is documented as a limitation in
+  [`docs/usage/fastapi.md`](../docs/usage/fastapi.md) (audit improvement
+  P18, #72). Forwarding the kwargs is small in `OutboxRouter.__init__` +
+  the `super().__init__` passthrough, but it's a real feature with a
+  design surface (defaults, AsyncAPI/typing implications, whether
+  `routers` even makes sense through the FastAPI lifespan), not a
+  mechanical fix — hence a spec, not a drive-by. Revisit when a concrete
+  "FastAPI + DLQ" or "FastAPI + recorder seam" demand surfaces.
+  (`faststream_outbox/fastapi/router.py`)