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/workers.py

@@ -1,3 +1,4 @@
+import ctrl.db as db
 import ctrl.settings as settings
 import requests
 
@@ -11,7 +12,14 @@ def parse_worker(spec):
 
 
 def configured_workers():
-    return [parse_worker(s) for s in settings.WORKERS]
+    out = []
+    for w in db.list_workers():
+        out.append({
+            "url": f"http://{w['host']}:{w['port']}",
+            "host": w["host"],
+            "port": w["port"],
+        })
+    return out
 
 
 def probe_worker(url, timeout=None):