Load tests with k6, for what many people at once do to the server (#133)

The browser tests drive one person against a handful of feeds, one request at a time, and cannot
show what production's load does. node tests/load/run.js builds a release binary, serves 1,500
generated feeds from itself, starts a daemon of its own under /tmp/ipx-load, seeds a hundred
listeners with thirty feeds each and four downloaded files, and runs four k6 scripts, with
`ipx status` -- the healthcheck -- run every second and failing the run at its 5s timeout:

  browse     25 people at once: feed list, All Subscriptions, a feed, a search, the Directory,
             a feed's page in it
  listening  100 players saving positions every second and marking items read while scans write;
             each reads its own state back, which must be as it left it
  media      50 listeners seeking: every range checked byte for byte against the served file
  signin     a flood of wrong passwords while others browse, and the gap between refusing a
             known name and an unknown one

Each budget is about twice what it measures now. Getting here found #135 (SQLite waiting for the
disk after every write, a first scan of 1,500 feeds estimated at 50 minutes, now 27s), #136
(SQLite's single connection, left open), #137 and #138 (fixed in the commit before this). k6 is
installed from its own signed apt repository by /src/install.sh, outside this repository.

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

tests/load/browse.js

@@ -0,0 +1,53 @@
+import http from 'k6/http';
+import { check, sleep } from 'k6';
+import { BASE, FEEDS, ITEMS, as, me, pick, feedId } from './lib.js';
+
+// An evening's browsing: twenty-five people at once opening their feed list, All Subscriptions,
+// a feed, a search, the Directory and a feed's page in it. Each answer's budget is well above what
+// it takes now, so it fails on a query that has started asking once per feed -- the Directory
+// asked the database three questions a feed until 0.10.0 -- not on a slow minute. Fifty measured
+// the queue for SQLite's one connection (#136) more than any query.
+export const options = {
+  scenarios: {
+    evening: {
+      executor: 'ramping-vus',
+      stages: [{ duration: '15s', target: 25 }, { duration: '45s', target: 25 }, { duration: '10s', target: 0 }],
+    },
+  },
+  thresholds: {
+    http_req_failed: ['rate==0'],
+    checks: ['rate==1'],
+    // About twice each one's p95 on 2026-10-05, SQLite on Tower: 236, 118, 133, 119, 259, 312ms.
+    'http_req_duration{name:feeds}': ['p(95)<500'],
+    'http_req_duration{name:all-entries}': ['p(95)<300'],
+    'http_req_duration{name:feed-entries}': ['p(95)<300'],
+    'http_req_duration{name:search}': ['p(95)<300'],
+    'http_req_duration{name:directory}': ['p(95)<600'],
+    'http_req_duration{name:listed}': ['p(95)<700'],
+  },
+};
+
+export default function () {
+  const u = me();
+  const feeds = http.get(`${BASE}/api/feeds`, as(u, 'feeds'));
+  check(feeds, { 'your thirty feeds': r => r.status === 200 && r.json().length === 30 });
+
+  const all = http.get(`${BASE}/api/entries?limit=50&filter=all&sort=published&dir=desc`, as(u, 'all-entries'));
+  check(all, { 'All Subscriptions, a page of it': r => r.status === 200 && r.json().entries.length === 50 });
+
+  const f = pick(feeds.json());
+  const one = http.get(`${BASE}/api/feeds/${f.id}/entries?limit=50&sort=title&dir=asc`, as(u, 'feed-entries'));
+  check(one, { 'a feed of yours, every item': r => r.status === 200 && r.json().entries.length === ITEMS });
+
+  // Every generated item mentions the weather, in its text, not its title.
+  const found = http.get(`${BASE}/api/entries?q=weather&limit=50`, as(u, 'search'));
+  check(found, { 'a search of everything you read': r => r.status === 200 && r.json().total === 30 * ITEMS });
+
+  const dir = http.get(`${BASE}/api/directory`, as(u, 'directory'));
+  check(dir, { 'the whole Directory': r => r.status === 200 && r.json().length === FEEDS });
+
+  const listed = http.get(`${BASE}/api/directory/${feedId(Math.floor(Math.random() * FEEDS))}`, as(u, 'listed'));
+  check(listed, { "a feed's page in the Directory": r => r.status === 200 && r.json().items.length === ITEMS });
+
+  sleep(1 + Math.random() * 2);
+}