fix: check Redis connectivity in /health

backend/src/monitoring/health.service.ts

@@ -1,5 +1,7 @@
-import { Injectable, Optional } from '@nestjs/common';
+import { Inject, Injectable, Optional } from '@nestjs/common';
+import type { Redis } from 'ioredis';
 import { DatabaseService } from '../common/database/database.service';
+import { REDIS_CLIENT } from '../common/redis/redis.module';
 
 export interface HealthStatus {
   status: 'ok' | 'degraded' | 'down';
@@ -11,13 +13,17 @@ export interface HealthStatus {
 export class HealthService {
   private readonly startTime = Date.now();
 
-  constructor(@Optional() private readonly database?: DatabaseService) {}
+  constructor(
+    @Optional() private readonly database?: DatabaseService,
+    @Optional() @Inject(REDIS_CLIENT) private readonly redis?: Redis | null,
+  ) {}
 
   async check(): Promise<HealthStatus> {
     const checks: Record<string, boolean> = {
       api: true,
       stellar: await this.checkStellar(),
       database: await this.checkDatabase(),
+      redis: await this.checkRedis(),
       memory: process.memoryUsage().heapUsed < 500 * 1024 * 1024,
     };
     const failing = Object.values(checks).filter(v => !v).length;
@@ -28,6 +34,22 @@ export class HealthService {
     };
   }
 
+  /**
+   * Redis is required infrastructure for rate limiting, idempotency keys, and
+   * the outbox — closes #219. An unconfigured REDIS_URL still reports healthy
+   * (this environment doesn't require Redis), but once configured a failed
+   * ping counts against overall health instead of being silently invisible.
+   */
+  private async checkRedis(): Promise<boolean> {
+    if (!this.redis) return true;
+    try {
+      const pong = await this.redis.ping();
+      return pong === 'PONG';
+    } catch {
+      return false;
+    }
+  }
+
   /**
    * PostgreSQL is currently optional infrastructure — not yet a hard dependency of any
    * service — so an unconfigured pool reports healthy (true) rather than degraded. Once