fix(ai): 队列会把共用的网关打到 502，加并发上限和间隔

排查生产上一直生成不出来，发现网关在返 502。上去看：机器好好的、systemd
说 active、5100 端口在监听——但它是 `gunicorn -w 1 --threads 4`，全部并发
就四个，而且 fam-edge 和摄像头项目也在用同一个。

我们这边一个请求占一个线程 2~5 分钟，NVIDIA 链重试起来最坏十七分钟（它自己
README 已知问题 #3）。而我写的 worker 是跑完一个立刻拉下一个，同步后还有八
个 scope 排队——等于拿满线程不撒手。这个 502 大概率是我打出来的，而且顺带
把另外两个项目也打下线了。

- 并发按整个部署计算，不是每个 gunicorn worker 一个：claim 前先数全局
  running（两个 worker 各跑「一个」就是两个并发）
- 每跑完一个任务停 20 秒，不只是空闲时才停
- 两个都可用环境变量调，注释里写清楚调大的代价是什么

补齐的历史数据晚二十分钟到没有任何人受影响；网关不响应是三个项目一起受影响。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/tests/test_coach.py

@@ -1073,3 +1073,35 @@ class TestHighlightsReadAlone:
                 f"{name}: 折叠态只显示 detail，但它没提到 {title!r}: "
                 f"{first['detail']!r}"
             )
+
+
+class TestGatewayCourtesy:
+    """The gateway runs one worker with four threads and is shared with two
+    other projects. This consumer must not be able to saturate it."""
+
+    def test_only_one_job_runs_at_a_time_across_the_deployment(self, db, user):
+        jobs.enqueue(user["id"], "health", "a")
+        jobs.enqueue(user["id"], "sleep", "b")
+        assert jobs._claim_next() is not None
+        assert jobs._claim_next() is None, \
+            "a second Gunicorn worker must not start a second gateway call"
+
+    def test_a_finished_job_frees_the_slot(self, db, user):
+        jobs.enqueue(user["id"], "health", "a")
+        jobs.enqueue(user["id"], "sleep", "b")
+        first = jobs._claim_next()
+        jobs._finish(first["id"])
+        assert jobs._claim_next() is not None
+
+    def test_an_abandoned_claim_does_not_block_the_queue_forever(self, db, user):
+        jobs.enqueue(user["id"], "health", "a")
+        jobs.enqueue(user["id"], "sleep", "b")
+        jobs._claim_next()
+        stale = jobs._now() - jobs.datetime.timedelta(
+            seconds=jobs.CLAIM_TIMEOUT_SECONDS + 60)
+        db.execute("UPDATE ai_jobs SET claimed_at = ?", [jobs._iso(stale)])
+        assert jobs._claim_next() is not None
+
+    def test_there_is_a_gap_between_jobs(self):
+        """Back-to-back is what saturates a four-thread box."""
+        assert jobs.GAP_SECONDS > 0