fix(test): stop the suite reaching a real database, and make it exit

`npm test` never terminated. Twenty-two test files omitted the two lines that
point the pool at a dead port, so utils/db.js -- which builds its mariadb pool at
require time and calls dotenv.config() itself -- picked up server/.env and opened
five live connections to the developer's MariaDB. The tests still passed, because
they stub their models and never issue a query; the only symptoms were a process
that never exited and five connections held for as long as it lived. Thirty
stranded workers is 150 connections, which is the whole server's limit, and that
is the "too many connections" this workspace has hit before.

The convention was right and only ever as good as the next test file's memory of
it, so it moves into the harness: test/_setup.js is loaded with --require by the
npm script, ahead of the test file it hosts, which is the only moment early
enough to matter. It pins the dead port -- dotenv does not overwrite an existing
variable, so an explicit DB_PORT= still wins for anyone who wants a live database
-- and closes the pool after the file's tests, so the process exits at once
instead of waiting out the driver's connect retries. The per-file preambles stay:
they keep `node --test test/one.test.js` safe on its own.

Two supporting fixes:

- db.close() is idempotent. pool.end() throws "pool is already closed" on a
  second call, and closing twice is now normal rather than exceptional -- the
  harness closes the pool for every file on top of the suites that close it
  themselves, and a SIGINT followed by a SIGTERM already reached the shutdown
  handler twice.
- test/_helper.js's close() destroys open connections. server.close() only stops
  accepting and waits for existing connections to end, and node's global fetch
  keeps its sockets alive, so the listener outlived the test that created it --
  invisible until now, because the pool was holding the process open anyway.

announceJobs.test.js alone: 120s+ hang -> 0.35s. The whole suite now finishes in
~75s where it previously did not finish at all: 901 tests, 901 pass, verified
three times on CI's exact platform (node:20 on Linux, via Docker).

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

server/test/_helper.js

@@ -14,7 +14,14 @@ async function startApp(configure) {
   const { port } = server.address()
   return {
     url: `http://127.0.0.1:${port}`,
-    close: () => new Promise((resolve) => server.close(resolve)),
+    // `server.close()` stops accepting and waits for open connections to end on
+    // their own — and node's global fetch keeps its sockets alive, so nothing
+    // ever ends them. The listener then outlives the test that made it, which
+    // used to be invisible because the pool held the process open anyway.
+    close: () => new Promise((resolve) => {
+      server.closeAllConnections()
+      server.close(resolve)
+    }),
   }
 }