Refactor nginx to path-based domain model with config migration

Replace the legacy top-level management key with a unified paths-based
model. Each domain now contains a paths map where each entry defines its
own backend, auth, headers, and flags (is_management, is_websocket).

- Add _migrate_config() to auto-migrate legacy formats on first load
- Remove set_management_proxy() and POST_NGINX_MANAGEMENT endpoint
- Update server_block.conf template to iterate paths with per-location auth
- Update daemon handler, API blueprint, state collector, and install script
- Add server config generation tests for paths, WebSocket, auth inheritance
- Update frontend proxy page to display per-path rows with flags

tests/test_api.py

@@ -766,21 +766,6 @@ class TestDhcpConfigCrud:
 # ============================================================================
 
 
-class TestProxyManagement:
-    @_px("post")
-    def test_set_management(self, mock_post, client):
-        mock_post.return_value = {"domain": "vacuum-wall.local"}
-        resp = client.post(
-            "/api/proxy/management",
-            json={
-                "domain": "vacuum-wall.local",
-                "flask_host": "127.0.0.1",
-                "flask_port": 9090,
-            },
-        )
-        assert resp.status_code == 200
-
-
 # ============================================================================
 # Proxy domain update
 # ============================================================================