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/lib.js

@@ -13,4 +13,7 @@ export const as = (name, tag) => ({
 /// The listener this virtual user is, one of the hundred run.js seeded, thirty feeds each.
 export const me = () => `load-${(__VU - 1) % USERS}`;
 export const pick = a => a[Math.floor(Math.random() * a.length)];
+/// A p95 budget, in ms, for whichever database the daemon is on (run.js --postgres). Postgres's
+/// are tighter: SQLite's carry the queue for its one connection (#136).
+export const p95 = (sqlite, postgres) => [`p(95)<${__ENV.DB === 'postgres' ? postgres : sqlite}`];
 export const feedId = n => `gen-${String(n).padStart(4, '0')}`;