feat(notifications): wire publisher + async feature flag (GOO-173)

Phase 1 step 3 — make NotificationsPublisher available via DI and
introduce the NOTIFICATIONS_ASYNC_ENABLED flag that will gate the
listener cutover.

- NotificationsAsyncConfig: tiny injectable that reads
  NOTIFICATIONS_ASYNC_ENABLED (truthy: 1/true/yes/on, default disabled).
  Callers don't touch process.env directly; per-category rollout can be
  added later without churn in listeners.
- NotificationsModule providers/exports now include NotificationsPublisher
  and NotificationsAsyncConfig so listeners/handlers can inject them.
- Tests (14 specs on the flag + 4 on the publisher = 18 total green):
  * default disabled when unset
  * truthy parsing (1, true, TRUE, yes, on, padded)
  * falsy parsing (false, 0, no, off, empty, unknown)
  * describe() reports human-readable state

No call sites updated yet — next commit migrates the first listener
(PaymentCompletedListener pilot) onto the publisher with the flag check.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/notifications/infrastructure/services/notifications-async.config.ts

@@ -0,0 +1,33 @@
+/**
+ * Feature-flag config for the Phase 1 async notifications path (RFC-004 / GOO-173).
+ *
+ * When NOTIFICATIONS_ASYNC_ENABLED is `true`, producers route through
+ * `NotificationsPublisher` → outbox → Redis Streams → BullMQ consumer.
+ * When `false` (default until parity is confirmed in production), producers
+ * continue to execute `SendNotificationCommand` directly in-process.
+ *
+ * Kept as a tiny injectable so:
+ *   - callers don't read process.env directly (testability)
+ *   - we have one place to evolve the flag shape (per-category rollout,
+ *     percentage shadowing, etc.) without ripping through listeners again
+ */
+
+import { Injectable } from '@nestjs/common';
+
+const FLAG_ENV = 'NOTIFICATIONS_ASYNC_ENABLED';
+
+@Injectable()
+export class NotificationsAsyncConfig {
+  /** True when the async outbox path should be used for new notifications. */
+  get asyncEnabled(): boolean {
+    const raw = process.env[FLAG_ENV];
+    if (raw === undefined) return false;
+    const v = raw.trim().toLowerCase();
+    return v === '1' || v === 'true' || v === 'yes' || v === 'on';
+  }
+
+  /** Exposed for observability / startup logs. */
+  describe(): string {
+    return `${FLAG_ENV}=${this.asyncEnabled ? 'enabled' : 'disabled'}`;
+  }
+}