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/db.py

@@ -41,6 +41,39 @@ def init_db():
         )
         """
     )
+    conn.execute(
+        """
+        CREATE TABLE IF NOT EXISTS workers (
+            host       TEXT NOT NULL,
+            port       INTEGER NOT NULL,
+            created_at TEXT NOT NULL,
+            PRIMARY KEY (host, port)
+        )
+        """
+    )
+    conn.commit()
+
+
+# ---- workers -------------------------------------------------------------
+
+def list_workers():
+    rows = _conn().execute("SELECT host, port FROM workers ORDER BY host, port").fetchall()
+    return [{"host": r["host"], "port": r["port"]} for r in rows]
+
+
+def add_worker(host, port):
+    import datetime
+    conn = _conn()
+    conn.execute(
+        "INSERT OR IGNORE INTO workers (host, port, created_at) VALUES (?, ?, ?)",
+        (host, int(port), datetime.datetime.now().isoformat()),
+    )
+    conn.commit()
+
+
+def remove_worker(host, port):
+    conn = _conn()
+    conn.execute("DELETE FROM workers WHERE host = ? AND port = ?", (host, int(port)))
     conn.commit()