feat: implement per-symbol cooldown period

CRITICAL: Cooldown was global across ALL symbols, causing missed opportunities
Example: ETH trade at 10:00 blocked SOL trade at 10:04 (5min cooldown)

Changes:
- Added getLastTradeTimeForSymbol() function to query last trade per symbol
- Updated check-risk endpoint to use symbol-specific cooldown
- Each coin (SOL/ETH/BTC) now has independent cooldown timer
- Cooldown message shows symbol: 'Must wait X min before next SOL-PERP trade'

Result: Can trade ETH and SOL simultaneously without interference
Example: ETH LONG at 10:00, SOL SHORT at 10:01 = both allowed

lib/database/trades.ts

@@ -276,6 +276,26 @@ export async function getLastTradeTime(): Promise<Date | null> {
   }
 }
 
+/**
+ * Get the most recent trade time for a specific symbol
+ */
+export async function getLastTradeTimeForSymbol(symbol: string): Promise<Date | null> {
+  const prisma = getPrismaClient()
+  
+  try {
+    const lastTrade = await prisma.trade.findFirst({
+      where: { symbol },
+      orderBy: { entryTime: 'desc' },
+      select: { entryTime: true },
+    })
+    
+    return lastTrade?.entryTime || null
+  } catch (error) {
+    console.error(`❌ Failed to get last trade time for ${symbol}:`, error)
+    return null
+  }
+}
+
 /**
  * Get the most recent trade with full details
  */