feat(quota): add spendRecorder fire-and-forget wrapper (B/F7)

scheduleRecordConsumption() wraps recordConsumption() in setImmediate so it
never adds latency to the client response path. Errors are caught and logged
via pino warn but NEVER propagated to the caller (B29 fail-open contract).

src/lib/quota/spendRecorder.ts

@@ -0,0 +1,42 @@
+/**
+ * spendRecorder.ts — Fire-and-forget wrapper for POST-response consumption.
+ *
+ * Schedules `recordConsumption` on the next event-loop tick via `setImmediate`
+ * so it never adds latency to the client response path.
+ *
+ * Errors from `recordConsumption` are caught and logged via pino (if a logger
+ * is provided) but NEVER propagated — per B29, drift is acceptable and will
+ * self-correct through the global saturation signal on the next request.
+ *
+ * Part of: Group B — Quota Sharing Engine (plan 22, frente F7).
+ */
+
+import { recordConsumption } from "./enforce";
+import type { RecordConsumptionInput } from "./types";
+
+// Minimal pino-compatible logger surface (only warn is needed)
+interface MinimalLogger {
+  warn?: (data: unknown, msg?: string) => void;
+}
+
+/**
+ * Schedule `recordConsumption` for the next event-loop tick.
+ *
+ * @param input  Consumption data to record.
+ * @param log    Optional pino logger; if omitted, errors are silently discarded.
+ */
+export function scheduleRecordConsumption(
+  input: RecordConsumptionInput,
+  log?: MinimalLogger | null
+): void {
+  setImmediate(() => {
+    recordConsumption(input).catch((err: unknown) => {
+      if (log?.warn) {
+        log.warn(
+          { err: err instanceof Error ? err.message : String(err) },
+          "[quotaShare] recordConsumption failed (drift expected)"
+        );
+      }
+    });
+  });
+}