feat: dedicated Instance Settings modal with admin controls

Move instance-level administration out of Space Settings into its own
modal. Adds admin UI for instance name, registration toggle, and
discovery toggle. Streaming limits panel relocated from SpaceSettings.

- Add InstanceAdminSettings type and GET/PATCH /api/settings/instance
- Add registration_open DB column (nullable, env var fallback)
- Auth registration and instance info now check DB override
- New InstanceSettings modal with General and Streaming tabs
- Admin shield button in UserAreaPanel (visible to admins only)
- Remove Streaming tab from SpaceSettings

packages/server/src/routes/instance.ts

@@ -13,10 +13,15 @@ export async function instanceRoutes(app: FastifyInstance): Promise<void> {
     const settings = db.select().from(schema.instanceSettings).where(eq(schema.instanceSettings.id, 1)).get();
     const instanceName = settings?.instanceName ?? 'Backspace';
 
+    // DB setting overrides env var if explicitly set by admin
+    const registrationOpen = settings?.registrationOpen !== null && settings?.registrationOpen !== undefined
+      ? settings.registrationOpen === 1
+      : config.registrationOpen;
+
     const response: InstanceInfoResponse = {
       name: instanceName,
       version: BACKSPACE_VERSION,
-      registrationOpen: config.registrationOpen,
+      registrationOpen,
     };
 
     return reply.code(200).send(response);