test: wait for the server to LISTEN instead of a fixed 300ms

Cold Postgres boot (connect + apply the full schema) takes a few seconds — the
fixed 300ms that was fine for SQLite's instant in-memory init raced the server
bind and the first fetch hit ECONNREFUSED. Poll BASE/ until it responds (≤30s).

Validated on local PostgreSQL 16: db-smoke 22/22 pass; e2e passes all DB-backed
checks (auth, messages, polls, groups, calls+invite, meetings) then stops at the
known pre-existing WS meeting-joined flake (unrelated to the DB).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/test/db-smoke.js

@@ -41,7 +41,9 @@ async function get(p, cookie) {
 }
 
 (async () => {
-  await wait(300);
+  // Wait for the server to actually be LISTENING — a cold Postgres boot (connect + apply the full schema)
+  // takes a few seconds, well past the fixed 300ms that was fine for SQLite's instant in-memory init.
+  for (let i = 0; i < 150; i++) { try { await fetch(BASE + '/'); break; } catch (_) { await wait(200); } }
   console.log('DB smoke tests (backend=' + process.env.DB_BACKEND + '):');
 
   // Auth