feat(plugins): plugin system scaffold — registry tables + admin panel

First slice of the plugin system. Lays down the data model and a read-only
admin surface; nothing executes yet.

- Migration 155: plugins, plugin_meta_migrations, plugin_error_log and
  plugin_settings_fields tables. Plugin data will live in a per-plugin sqlite
  file under /plugins-data, never in these tables.
- New Nest module server/src/nest/plugins with GET /api/admin/plugins
  (admin-gated, returns the installed list + the runtime-enabled flag).
- TREK_PLUGINS_ENABLED kill switch (config.pluginsEnabled), off by default.
- Admin → Plugins tab with a read-only panel: installed list, status badges,
  and a clear banner when the runtime is disabled by server config.
- i18n keys for the tab and panel across all locales.

Install, activation, the isolated runtime and the registry browser follow in
later slices.

server/src/nest/plugins/plugins.service.ts

@@ -0,0 +1,36 @@
+import { Injectable } from '@nestjs/common';
+import { db } from '../../db/database';
+import { pluginsEnabled } from '../../config';
+
+/**
+ * Read side of the plugin system (#plugins), M0 scaffold. Lists installed
+ * plugins from the `plugins` registry table and reports whether the runtime is
+ * enabled (TREK_PLUGINS_ENABLED). No execution here — the isolated runtime,
+ * install pipeline and registry fetch land in later milestones.
+ */
+
+export interface PluginListItem {
+  id: string;
+  name: string;
+  description: string | null;
+  type: string;
+  icon: string | null;
+  version: string | null;
+  status: string;
+  reviewed_at: string | null;
+  source_repo: string | null;
+}
+
+@Injectable()
+export class PluginsService {
+  list(): { enabled: boolean; plugins: PluginListItem[] } {
+    const plugins = db
+      .prepare(
+        `SELECT id, name, description, type, icon, version, status, reviewed_at, source_repo
+         FROM plugins
+         ORDER BY sort_order, name`,
+      )
+      .all() as PluginListItem[];
+    return { enabled: pluginsEnabled(), plugins };
+  }
+}