feat: Wire NATS command publishing for server commands and plugin installs

- servers.service: sendCommand() now throws InternalServerErrorException on
  NATS failure instead of silently succeeding; returns { success, message }
  instead of the legacy { output } shape; adds NestJS Logger
- plugins.service: installPlugin() dispatches plugin_install to
  corrosion.{license_id}.cmd.server after DB save; NATS failure is logged
  but non-fatal so the DB record is preserved regardless

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-nest/src/modules/plugins/plugins.service.ts

@@ -45,7 +45,21 @@ export class PluginsService {
       is_loaded: false,
     });
 
-    return this.pluginRegistryRepo.save(plugin);
+    const saved = await this.pluginRegistryRepo.save(plugin);
+
+    try {
+      await this.natsService.publish(`corrosion.${licenseId}.cmd.server`, {
+        action: 'plugin_install',
+        plugin_name: dto.plugin_name,
+        umod_slug: dto.umod_slug,
+        timestamp: new Date().toISOString(),
+      });
+      this.logger.log(`Plugin install dispatched for ${dto.plugin_name} on license ${licenseId}`);
+    } catch (err) {
+      this.logger.error(`Failed to dispatch plugin install for ${dto.plugin_name} on license ${licenseId}: ${(err as Error).message}`);
+    }
+
+    return saved;
   }
 
   async uninstallPlugin(licenseId: string, pluginId: string): Promise<void> {