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/templates/settings/plugins.html

@@ -76,7 +76,8 @@
 <div class="section-title" style="margin-bottom:0.4rem;">Integrations</div>
 <p style="color:var(--text-muted);font-size:0.82rem;margin:0 0 0.75rem;max-width:720px;">
   External systems Steward pulls data from (e.g. Traefik, UniFi). Enable and configure each one
-  to surface its own dashboard. Off until you set them up.
+  to surface its own dashboard. Off until you set them up. A plugin of your own goes in
+  <code>/data/plugins</code>; restart and it appears here.
 </p>
 {% if integrations %}
 <div style="display:grid;gap:0.6rem;max-width:720px;margin-bottom:1.5rem;">
@@ -84,7 +85,7 @@
 </div>
 {% else %}
 <div class="card" style="color:var(--text-muted);font-size:0.9rem;margin-bottom:1.5rem;">
-  No integrations installed. Browse the catalog below.
+  No integrations installed.
 </div>
 {% endif %}
 
@@ -97,8 +98,8 @@
   <p style="color:var(--text-muted);font-size:0.82rem;margin:0 0 0.75rem;">
     Plugins Steward has settings for, or tried to load, but cannot find on disk — including
     anything counted by the "failed to load" banner. This usually means the plugin was
-    removed, but it also happens when an external plugin directory isn't mounted or an
-    install failed part-way, so nothing is deleted automatically. Removing an entry
+    removed, but it also happens when an external plugin directory isn't mounted, so
+    nothing is deleted automatically. Removing an entry
     discards that plugin's saved configuration, including any credentials.
   </p>
   <div style="display:grid;gap:0.6rem;">
@@ -145,40 +146,4 @@
 </div>
 {% endif %}
 
-{# ── Plugin Repositories ───────────────────────────────────────────────────── #}
-<div style="margin-bottom:2rem;max-width:720px;">
-  <div class="section-title" style="margin-bottom:0.75rem;">Plugin Repositories</div>
-  <p style="color:var(--text-muted);font-size:0.84rem;margin-bottom:0.75rem;">
-    Repositories are remote <code>index.yaml</code> files that Steward checks for available
-    and updated plugins. Add repos from other developers to expand the plugin catalog.
-  </p>
-  {% include "settings/_plugin_repos.html" %}
-</div>
-
-{# ── Plugin Catalog ────────────────────────────────────────────────────────── #}
-<div style="max-width:720px;">
-  <div style="display:flex;align-items:center;justify-content:space-between;margin-bottom:0.75rem;">
-    <div class="section-title">Plugin Catalog</div>
-    <button class="btn btn-ghost btn-sm" style="font-size:0.78rem;"
-            hx-post="/settings/plugins/catalog/refresh"
-            hx-target="#plugin-catalog"
-            hx-swap="outerHTML">
-      Refresh
-    </button>
-  </div>
-
-  {% if repos %}
-  <div id="plugin-catalog"
-       hx-get="/settings/plugins/catalog/"
-       hx-trigger="load"
-       hx-swap="outerHTML">
-    <div style="color:var(--text-muted);font-size:0.85rem;padding:0.5rem 0;">Loading catalog…</div>
-  </div>
-  {% else %}
-  <div id="plugin-catalog" style="color:var(--text-muted);font-size:0.85rem;padding:0.5rem 0;">
-    Add a plugin repository above to browse available plugins.
-  </div>
-  {% endif %}
-</div>
-
 {% endblock %}