FRE-4521 Implement Redis integration for rate limiting and deduplication

- Add ioredis dependency for Redis connection pooling
- Create RedisService singleton with connection management
- Add Redis config (url, dedupWindowSeconds) to notification.config.ts
- Implement NotificationService.checkRateLimit using Redis INCR+EXPIRE
- Implement NotificationService.deduplicateNotification using Redis SET/NX
- Add configurable rate limit windows and thresholds via env vars
- Add 29 unit tests covering Redis operations, rate limiting, and dedup
- All tests pass, TypeScript compiles cleanly for new files

packages/shared-notifications/src/services/redis.service.ts

@@ -0,0 +1,73 @@
+import { Redis, RedisOptions } from 'ioredis';
+
+export interface RedisServiceConfig {
+  url?: string;
+  options?: RedisOptions;
+}
+
+export class RedisService {
+  private static instance: RedisService;
+  private client: Redis;
+
+  private constructor(config?: RedisServiceConfig) {
+    const url = config?.url || process.env.REDIS_URL || 'redis://localhost:6379';
+    this.client = new Redis(url, config?.options || {});
+  }
+
+  static getInstance(config?: RedisServiceConfig): RedisService {
+    if (!RedisService.instance) {
+      RedisService.instance = new RedisService(config);
+    }
+    return RedisService.instance;
+  }
+
+  getClient(): Redis {
+    return this.client;
+  }
+
+  async isConnected(): Promise<boolean> {
+    return this.client.status === 'ready';
+  }
+
+  async increment(key: string, expirySeconds?: number): Promise<number> {
+    const current = await this.client.incr(key);
+    if (current === 1 && expirySeconds) {
+      await this.client.expire(key, expirySeconds);
+    }
+    return current;
+  }
+
+  async getCounter(key: string): Promise<number> {
+    const value = await this.client.get(key);
+    return value ? parseInt(value, 10) : 0;
+  }
+
+  async setWithExpiry(key: string, value: string, seconds: number): Promise<'OK' | 1> {
+    return this.client.setex(key, seconds, value);
+  }
+
+  async setIfNotExists(key: string, value: string, seconds: number): Promise<boolean> {
+    const result = await this.client.set(key, value, 'EX', seconds, 'NX');
+    return result === 'OK';
+  }
+
+  async get(key: string): Promise<string | null> {
+    return this.client.get(key);
+  }
+
+  async ttl(key: string): Promise<number> {
+    return this.client.ttl(key);
+  }
+
+  async delete(key: string): Promise<number> {
+    return this.client.del(key);
+  }
+
+  async getTTL(key: string): Promise<number> {
+    return this.client.ttl(key);
+  }
+
+  async close(): Promise<void> {
+    await this.client.quit();
+  }
+}