Manage workers from the web UI, persisted in SQLite

Replace the CROSS_PY_WORKERS env list with a DB-backed worker store so agents
can be added/removed (and their online status verified) from the browser.
- db.py: add workers table + list_workers/add_worker/remove_worker.
- workers.py: configured_workers() reads the DB instead of settings.
- app.py: POST /api/workers (add, probes reachability) and
  DELETE /api/workers/<port>/<host>; startup log uses DB worker count.
- UI: Workers card with add form, per-worker live status + OS/CPU, remove.
- settings.py: drop CROSS_PY_WORKERS/_get_json_list; DB is single source.
- README/DESIGN updated to describe UI-managed workers.

ctrl/static/index.html

@@ -13,6 +13,15 @@
   </header>
 
   <main>
+    <section class="card">
+      <h2>Workers</h2>
+      <form id="worker-form" class="inline-form">
+        <input type="text" id="worker-spec" placeholder="host:port (e.g. 192.168.1.40:9001)">
+        <button type="submit">Add worker</button>
+      </form>
+      <div id="workers"></div>
+    </section>
+
     <section class="card" id="submit-card">
       <h2>New build</h2>
       <form id="job-form">