Add configurable tick interval (time between trades)

- Seed tick_interval_seconds setting (default 60s)
- Expose tick_interval_seconds via GET/PATCH /api/settings
- startScheduler reads the setting at start time; enforces 5s minimum
- getSchedulerStatus returns intervalSeconds for the UI
- Main page: "Every [__] s" input in idle bar — saves on blur, persists across
  restarts; running state displays "every Ns" next to symbol/action

Co-Authored-By: Claude Sonnet 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'] as const;
+const VALID_KEYS = ['max_concurrent_accounts', 'tick_interval_seconds'] as const;
 type SettingKey = typeof VALID_KEYS[number];
 
 export async function GET() {