fix(setup): preserve existing ip_range when re-running make setup

setup_cell.py now reads ip_range from cell_config.json before falling
back to CELL_IP_RANGE env var, so re-running make setup on an existing
install doesn't reset the .env subnet to the default 172.20.0.0/16.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/setup_cell.py

@@ -205,12 +205,23 @@ def write_compose_env(ip_range: str):
         print(f'[WARN]    Could not write .env — containers will use built-in default IPs/ports')
 
 
+def _read_existing_ip_range() -> str:
+    """Read ip_range from existing cell_config.json if present, else return None."""
+    cfg_path = os.path.join(ROOT, 'config', 'api', 'cell_config.json')
+    try:
+        existing = json.loads(open(cfg_path).read())
+        return existing.get('_identity', {}).get('ip_range') or None
+    except Exception:
+        return None
+
+
 def main():
     cell_name = os.environ.get('CELL_NAME', 'mycell')
     domain = os.environ.get('CELL_DOMAIN', 'cell')
     vpn_address = os.environ.get('VPN_ADDRESS', '10.0.0.1/24')
     wg_port = int(os.environ.get('WG_PORT', '51820'))
-    ip_range = os.environ.get('CELL_IP_RANGE', '172.20.0.0/16')
+    # Prefer existing config ip_range over env var so `make setup` is safe to re-run
+    ip_range = os.environ.get('CELL_IP_RANGE') or _read_existing_ip_range() or '172.20.0.0/16'
 
     print('--- Personal Internet Cell: Setup ---')
     print(f'    cell={cell_name}  domain={domain}  vpn={vpn_address}  port={wg_port}')