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/auth.ts

@@ -71,12 +71,17 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(400).send({ error: 'Password must be at least 6 characters', statusCode: 400 });
     }
 
-    if (!config.registrationOpen) {
+    const db = getDb();
+
+    // Check registration: DB setting overrides env var if explicitly set by admin
+    const instanceRow = db.select().from(schema.instanceSettings).where(eq(schema.instanceSettings.id, 1)).get();
+    const registrationOpen = instanceRow?.registrationOpen !== null && instanceRow?.registrationOpen !== undefined
+      ? instanceRow.registrationOpen === 1
+      : config.registrationOpen;
+    if (!registrationOpen) {
       return reply.code(403).send({ error: 'Registration is currently closed', statusCode: 403 });
     }
 
-    const db = getDb();
-
     const existing = db.select().from(schema.users).where(eq(schema.users.username, trimmedUsername)).get();
     if (existing) {
       return reply.code(409).send({ error: 'Username already taken', statusCode: 409 });