refactor(plugins): no plugin catalog; plugins ship in the image or are mounted (#5374)

First-party plugins have shipped inside the image since a7a281c, and the
catalog's only repository (Steward-plugins) answers 404, so Steward polled a
dead URL every minute for nothing. The operator chose to remove the catalog
entirely (#5214 "Review: readable and fast on the live instance").

Removed:
- core/plugin_index.py and the plugin_catalog_refresh startup task;
- the plugins.index_url default, and migration 0035 deletes the stored
  plugins.index_url / plugins.repositories rows;
- the Settings -> Plugins repository editor, catalog, refresh, install and
  reload routes and their templates;
- the download/checksum/extract install path in plugin_manager, and the
  PLUGIN_INSTALL_DIR config it wrote to;
- plugins/index.yaml, docs/plugins/index.yaml.example and the guide's
  "Publishing to the Catalog" section (now "Shipping a Plugin").

Kept: enabling and configuring plugins, hot-reload on enable, restart, orphan
cleanup, and the external root (/data/plugins) for operator-mounted plugins.
tests/test_no_plugin_catalog.py guards the routes, default and task.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_plugin_dirs.py

@@ -67,8 +67,6 @@ def test_bootstrap_splits_bundled_and_external(tmp_path, monkeypatch):
     boot = load_bootstrap(config_path=tmp_path / "absent.yaml")
 
     assert boot["plugin_dirs"] == ["plugins", "/srv/external-plugins"]
-    # Installs target the external (writable) root, never the bundled one.
-    assert boot["plugin_install_dir"] == "/srv/external-plugins"
 
 
 def test_bootstrap_external_defaults_to_data_plugins(tmp_path, monkeypatch):
@@ -81,4 +79,3 @@ def test_bootstrap_external_defaults_to_data_plugins(tmp_path, monkeypatch):
     boot = load_bootstrap(config_path=tmp_path / "absent.yaml")
 
     assert boot["plugin_dirs"] == ["plugins", "/data/plugins"]
-    assert boot["plugin_install_dir"] == "/data/plugins"