Make the test workflow wait for the API server to be ready

Original task (#127): the `test` workflow ran `pnpm --filter
@workspace/api-server test` directly, which fires HTTP requests at
localhost:8080. On a freshly-started environment the API server
isn't up yet, so every test fails with ECONNREFUSED, drowning real
failures in noise.

Changes:
- Added `artifacts/api-server/scripts/wait-for-server.mjs`, a small
  pure-Node poller that hits `${TEST_API_BASE ?? "http://localhost:8080"}/api/healthz`
  every 500ms until it returns `{status: "ok"}` or the timeout
  (default 30s) elapses. On timeout it exits 1 with a clear
  "Start the API Server workflow (or set TEST_API_BASE) before
  running tests" message instead of a wall of fetch errors.
  Configurable via `TEST_API_BASE`, `TEST_API_WAIT_TIMEOUT_MS`,
  `TEST_API_WAIT_INTERVAL_MS`.
- Added `test:wait` script to `artifacts/api-server/package.json`.
- Updated the `test` workflow to run
  `pnpm --filter @workspace/api-server test:wait` before the
  api-server tests and the tx-os e2e tests.

Verified:
- `node ./scripts/wait-for-server.mjs` against a running server
  reports "ready ... after 86ms" and exits 0.
- Same script with TEST_API_BASE pointed at a dead port exits 1
  with the friendly message.
- The full `test` workflow now flows past the readiness gate and
  runs all 158 tests; 153 pass, 3 skip, and 2 fail for real reasons
  (PDF export endpoints returning 500). Filed follow-up #179 for
  the PDF bugs.

No deviations from the task. `.replit` was edited via the workflow
configuration tool (direct edits are blocked).

artifacts/api-server/package.json

@@ -8,6 +8,7 @@
     "build": "node ./build.mjs",
     "start": "node --enable-source-maps ./dist/index.mjs",
     "typecheck": "tsc -p tsconfig.json --noEmit",
+    "test:wait": "node ./scripts/wait-for-server.mjs",
     "test": "node --test 'tests/**/*.test.mjs'"
   },
   "dependencies": {