feat(push-gateway,msg): redis pubsub backplane for real-time notifications

M7: ADR-040 Redis Pub/Sub as state routing backplane

- push-gateway: remove Kafka consumer, add SSE endpoint

- SSE: subscribe to Redis user:{userId}:notify on connect

- msg: publish notifications to Redis Pub/Sub instead of HTTP push

- docker-compose: remove Kafka env from push-gateway

services/msg/src/shared/push/redis-publisher.ts

@@ -0,0 +1,78 @@
+import { getRedis } from "../redis/redis.client.js";
+import { logger } from "../observability/logger.js";
+
+/**
+ * Redis Pub/Sub 实时推送发布器（M7 / ADR-040）。
+ *
+ * 仲裁依据：
+ * - M7：msg 服务消费 Kafka（持久化、脱敏）后，通过 Redis Pub/Sub 发布实时推送
+ * - 通道命名：`user:{userId}:notify`
+ * - realtime-gateway 实例仅在用户连接 SSE 时 SUBSCRIBE 该通道
+ * - 软失败：Redis 不可用时返回 { sent: false }，不阻断主流程
+ *
+ * 消息格式：JSON 字符串，由 realtime-gateway SSE 端点原样转发为 `data: {json}\n\n`
+ */
+
+export interface RedisPushRequest {
+  userId: string;
+  event: string;
+  data: Record<string, unknown>;
+}
+
+export interface RedisPushResult {
+  sent: boolean;
+  error?: string;
+}
+
+/**
+ * 构造 Redis Pub/Sub 通道名。
+ * 通道命名规范（M7）：`user:{userId}:notify`
+ */
+export function notifyChannel(userId: string): string {
+  return `user:${userId}:notify`;
+}
+
+/**
+ * 通过 Redis Pub/Sub 发布实时通知到用户的通道。
+ *
+ * 软失败语义：
+ * - REDIS_URL 未配置 → 返回 { sent: false }，不报错
+ * - Redis 发布错误 → 返回 { sent: false, error }，logger.warn
+ * - 成功 → 返回 { sent: true }
+ *
+ * 消息体为 JSON 字符串，realtime-gateway SSE 端点原样转发为 SSE data 帧。
+ */
+export async function publishToRedisPubSub(
+  req: RedisPushRequest,
+): Promise<RedisPushResult> {
+  const client = getRedis();
+  if (!client) {
+    return { sent: false };
+  }
+
+  const channel = notifyChannel(req.userId);
+  const payload = JSON.stringify({
+    event: req.event,
+    data: req.data,
+    timestamp: new Date().toISOString(),
+  });
+
+  try {
+    const receivers = await client.publish(channel, payload);
+    if (receivers === 0) {
+      // 无订阅者：用户未通过 SSE 连接，不算错误（站内信仍可见）
+      logger.debug(
+        { userId: req.userId, channel },
+        "Redis Pub/Sub: no active SSE subscribers (user offline)",
+      );
+    }
+    return { sent: true };
+  } catch (err) {
+    const error = err instanceof Error ? err.message : String(err);
+    logger.warn(
+      { err, userId: req.userId, channel },
+      "Redis Pub/Sub publish failed",
+    );
+    return { sent: false, error };
+  }
+}