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/settings.py

@@ -1,4 +1,3 @@
-import json
 import os
 
 
@@ -9,19 +8,6 @@ def _get_bool(name, default=False):
     return val.strip().lower() in ("1", "true", "yes", "on")
 
 
-def _get_json_list(name, default=None):
-    raw = os.environ.get(name)
-    if not raw:
-        return list(default or [])
-    try:
-        value = json.loads(raw)
-    except json.JSONDecodeError:
-        raise ValueError(f"{name} must be a JSON array string")
-    if not isinstance(value, list):
-        raise ValueError(f"{name} must be a JSON array")
-    return value
-
-
 def _get_int(name, default):
     raw = os.environ.get(name)
     if raw is None:
@@ -34,9 +20,6 @@ DB_PATH = os.environ.get("CROSS_PY_DB", os.path.join(DATA_DIR, "jobs.db"))
 BUILDS_DIR = os.environ.get("CROSS_PY_BUILDS", os.path.join(DATA_DIR, "builds"))
 PORT = _get_int("CROSS_PY_PORT", 8080)
 
-# Static worker list: JSON array of "host:port" strings.
-WORKERS = _get_json_list("CROSS_PY_WORKERS")
-
 # One job at a time by default; raise to allow parallel dispatches to distinct workers.
 MAX_CONCURRENT = _get_int("CROSS_PY_MAX_CONCURRENT", 1)