feat(content): eager invalidation and optimistic lock for cqrs

M4: ADR-038 Eager Invalidation + ADR-039 Optimistic Lock

- EagerInvalidationService: Redis DEL after MySQL commit

- Cache key conventions for textbook/chapter/kp/question

- Version header check (If-Match/X-Expected-Version) on write endpoints

- 409 Conflict on version mismatch

- All write endpoints return updatedAt timestamp

services/content/src/config/redis.ts

@@ -0,0 +1,44 @@
+import { Redis } from "ioredis";
+import type { Redis as RedisClient } from "ioredis";
+import { env } from "./env.js";
+
+// Redis Client 惰性初始化：未配置 REDIS_URL 时 client 保持 null，
+// 服务仍可正常启动。所有依赖 Redis 的功能（Eager Invalidation）在
+// client 为 null 时优雅降级（跳过失效，由 Kafka projector 兜底），
+// 不会阻塞主流程。
+let client: RedisClient | null = null;
+let initAttempted = false;
+
+export function getRedis(): RedisClient | null {
+  if (initAttempted) {
+    return client;
+  }
+  initAttempted = true;
+  if (!env.REDIS_URL) {
+    return null;
+  }
+  try {
+    client = new Redis(env.REDIS_URL, {
+      maxRetriesPerRequest: 1,
+      enableReadyCheck: true,
+      lazyConnect: false,
+    });
+  } catch (err) {
+    console.warn(
+      "Redis client init failed, running without Redis:",
+      err instanceof Error ? err.message : String(err),
+    );
+    client = null;
+  }
+  return client;
+}
+
+/**
+ * 关闭 Redis Client 连接。
+ */
+export async function closeRedis(): Promise<void> {
+  if (client) {
+    await client.quit();
+    client = null;
+  }
+}