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>

package.json

@@ -7,7 +7,8 @@
     "typecheck": "tsc -p .",
     "smoke": "node tests/page-smoke.js",
     "test": "playwright test",
-    "test:headed": "playwright test --headed"
+    "test:headed": "playwright test --headed",
+    "load": "node tests/load/run.js"
   },
   "devDependencies": {
     "@playwright/test": "^1.56.0",