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.module.ts

@@ -0,0 +1,14 @@
+import { Module } from '@nestjs/common';
+import { PluginsController } from './plugins.controller';
+import { PluginsService } from './plugins.service';
+
+/**
+ * Plugin system (#plugins). M0 registers only the read side; the isolated
+ * runtime supervisor, install pipeline and registry service are added in later
+ * milestones under this same module.
+ */
+@Module({
+  controllers: [PluginsController],
+  providers: [PluginsService],
+})
+export class PluginsModule {}