Add master dashboard reporter and settings UI

- New reporter module that pushes firm stats (total accounts, accounts
  traded, in trade) to a configurable master dashboard every 30 seconds
- Add Instance Name and Dashboard URL fields to the settings page
- Register master_dashboard_url and instance_name in settings API
- Seed default (empty) values for new settings in db

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

app/api/settings/route.ts

@@ -1,7 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { getSetting, setSetting } from '@/lib/db';
 
-const VALID_KEYS = ['max_concurrent_accounts', 'tick_interval_seconds'] as const;
+const VALID_KEYS = ['max_concurrent_accounts', 'tick_interval_seconds', 'master_dashboard_url', 'instance_name'] as const;
 type SettingKey = typeof VALID_KEYS[number];
 
 export async function GET() {