feat: presence on tickets

A ticket's page beats every 10 s (touchPresence / PUT
/issues/{issue}/presence, with whether I am editing) and gets back who
else is there, editing ones first. Entries live in Redis, one hash per
ticket, and fade after 30 s without a beat; leaving the page takes me out
at once. Only for people who can read the ticket; never in the event
store. The ticket shows «Bob está editando · Ana también lo está viendo»
above its title; editing means a ticket or comment form is open or a
comment is being written.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/infrastructure/src/lib/redis/redis-presence.store.ts

@@ -0,0 +1,57 @@
+import { Inject, Injectable } from '@nestjs/common';
+import { REDIS_CLIENT } from '@task-stream/api/shared/infrastructure';
+import {
+  PresenceEntry,
+  PresenceStore,
+} from '@task-stream/api/tracker/application';
+import Redis from 'ioredis';
+
+/**
+ * One hash per ticket (field: user id, value: the entry). The hash expires
+ * when nobody beats on it; stale fields are skipped and dropped on reading.
+ */
+@Injectable()
+export class RedisPresenceStore extends PresenceStore {
+  constructor(@Inject(REDIS_CLIENT) private readonly redis: Redis) {
+    super();
+  }
+
+  private key(issueId: string): string {
+    return `task-stream:presence:${issueId}`;
+  }
+
+  async touch(
+    issueId: string,
+    entry: PresenceEntry,
+    ttlMs: number,
+  ): Promise<void> {
+    const key = this.key(issueId);
+    await this.redis
+      .multi()
+      .hset(key, entry.user.id, JSON.stringify(entry))
+      .pexpire(key, ttlMs)
+      .exec();
+  }
+
+  async list(issueId: string, since: number): Promise<PresenceEntry[]> {
+    const key = this.key(issueId);
+    const all = await this.redis.hgetall(key);
+    const fresh: PresenceEntry[] = [];
+    const stale: string[] = [];
+    for (const [userId, raw] of Object.entries(all)) {
+      try {
+        const entry = JSON.parse(raw) as PresenceEntry;
+        if (Date.parse(entry.at) >= since) fresh.push(entry);
+        else stale.push(userId);
+      } catch {
+        stale.push(userId);
+      }
+    }
+    if (stale.length) await this.redis.hdel(key, ...stale);
+    return fresh;
+  }
+
+  async leave(issueId: string, userId: string): Promise<void> {
+    await this.redis.hdel(this.key(issueId), userId);
+  }
+}