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>

steward/migrations/versions/0035_drop_plugin_catalog_settings.py

@@ -0,0 +1,29 @@
+"""Drop the plugin catalog's stored settings
+
+Steward no longer has a plugin catalog (#5374): first-party plugins ship in the
+image and a third-party plugin is a folder the operator mounts, so nothing reads
+`plugins.index_url` or `plugins.repositories` any more. The removed feature
+takes its rows with it, as 0025 did for the http plugin.
+
+Revision ID: 0035_drop_plugin_catalog_settings
+Revises: 0034_user_time_zone
+Create Date: 2026-10-08
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "0035_drop_plugin_catalog_settings"
+down_revision: Union[str, None] = "0034_user_time_zone"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(sa.text(
+        "DELETE FROM app_settings WHERE key IN ('plugins.index_url', 'plugins.repositories')"))
+
+
+def downgrade() -> None:
+    # Deliberately empty: nothing would read the rows.
+    pass