Run the load tests on Postgres too, as production runs (#139)

node tests/load/run.js --postgres puts the scratch daemon in IPX_TEST_DATABASE_URL, the database
the Rust tests already use on Postgres, ipodderx_test on the databases project's server, in a
schema of its own, ipx_load, dropped and made again each run as /tmp/ipx-load is wiped. The URL
is in /src/.envrc, taken from ipx.env. It is production's server and role, so the harness refuses
a database whose name does not end in _test, and hands psql the password in its environment,
where no other process on Tower can read it. psql comes from /src/install.sh.

After seeding it runs ANALYZE: a schema filled seconds before has no planner statistics until
autovacuum gets there, which it did partway through a test, and the search's p95 swung between
40ms and 166ms from run to run. Analyzed first, three runs gave 37-38ms.

Each budget now has a value for each database, about twice what it measures on Tower: Postgres
answers the feed list at p95 54ms to SQLite's 242, having no one connection to queue for (#136).
Running it found #140.

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

tests/load/signin.js

@@ -17,7 +17,8 @@ export const options = {
   thresholds: {
     http_req_failed: ['rate==0'],
     checks: ['rate==1'],
-    // 57ms on 2026-10-05; 4.3s while password checks ran on the async workers (#137).
+    // 57ms on 2026-10-05 on SQLite, 68 on Postgres; 4.3s while password checks ran on the async
+    // workers (#137).
     'http_req_duration{name:meanwhile}': ['p(95)<300'],
     // Known name against unknown, one straight after the other: no gap but noise. 0.2ms on
     // 2026-10-05; 31ms while an unknown name was refused without a check (#138).