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>

lib/auto-trade.ts

@@ -336,8 +336,9 @@ export function startScheduler(action: 'Buy' | 'Sell' | 'Random', symbol: string
         }
     };
 
-    state.intervalId = setInterval(tick, 60_000);
-    console.log(`[scheduler] started — ${action} ${symbol} every 60s`);
+    const intervalSecs = Math.max(5, parseInt(getSetting('tick_interval_seconds') ?? '60', 10));
+    state.intervalId = setInterval(tick, intervalSecs * 1_000);
+    console.log(`[scheduler] started — ${action} ${symbol} every ${intervalSecs}s`);
 }
 
 export function stopScheduler() {
@@ -357,5 +358,6 @@ export function getSchedulerStatus() {
         action: state.action,
         symbol: state.symbol,
         lastRun: state.lastRun,
+        intervalSeconds: parseInt(getSetting('tick_interval_seconds') ?? '60', 10),
     };
 }