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/app.py

@@ -46,6 +46,36 @@ def api_workers():
     return jsonify(wrk.probe_all())
 
 
+@app.post("/api/workers")
+def api_add_worker():
+    data = request.get_json(silent=True) or {}
+    spec = str(data.get("spec") or "").strip()
+    host = str(data.get("host") or "").strip()
+    port = data.get("port")
+    try:
+        if spec:
+            w = wrk.parse_worker(spec)
+        elif host and port:
+            w = wrk.parse_worker(f"{host}:{int(port)}")
+        else:
+            return jsonify({"error": "Provide spec ('host:port') or host+port"}), 400
+    except (ValueError, TypeError) as e:
+        return jsonify({"error": str(e)}), 400
+
+    db.add_worker(w["host"], w["port"])
+    db.set_queued_jobs_for_worker(w["url"])
+    log.info("Added worker %s:%s", w["host"], w["port"])
+    status = wrk.probe_worker(w["url"])
+    return jsonify({"host": w["host"], "port": w["port"], "online": status is not None, "status": status}), 201
+
+
+@app.delete("/api/workers/<int:port>/<path:host>")
+def api_remove_worker(host, port):
+    db.remove_worker(host, port)
+    log.info("Removed worker %s:%s", host, port)
+    return jsonify({"removed": True, "host": host, "port": port}), 200
+
+
 @app.get("/api/capabilities")
 def api_capabilities():
     combos = {
@@ -161,7 +191,7 @@ def main():
     _ensure_ready()
     scheduler.start()
     _configure_logging()
-    log.info("Controller listening on 0.0.0.0:%s (%d workers configured)", settings.PORT, len(settings.WORKERS))
+    log.info("Controller listening on 0.0.0.0:%s (%d workers configured)", settings.PORT, len(db.list_workers()))
     if settings.DEBUG:
         app.run(host="0.0.0.0", port=settings.PORT, threaded=True)
         return