feat: switch from Helius to Alchemy RPC provider

Changes:
- Updated SOLANA_RPC_URL to use Alchemy (https://solana-mainnet.g.alchemy.com/v2/...)
- Migrated from Helius free tier to Alchemy free tier
- Includes previous rate limit fixes (8s backoff, 2s operation delays)

Context:
- Helius free tier: 10 req/sec sustained, 100 req/sec burst
- Alchemy free tier: 300M compute units/month (more generous)
- User hit 239 rate limit errors in 10 minutes on Helius
- User registered Alchemy account and provided API key

Impact:
- Should significantly reduce 429 rate limit errors
- Better free tier limits for trading bot operations
- Combined with delay fixes for optimal RPC usage

app/api/trading/execute/route.ts

@@ -299,6 +299,9 @@ export async function POST(request: NextRequest): Promise<NextResponse<ExecuteTr
     console.log(`   Leverage: ${leverage}x`)
     console.log(`   Total position: $${positionSizeUSD}`)
 
+    // Helper function for rate limit spacing
+    const rpcDelay = (ms: number) => new Promise(resolve => setTimeout(resolve, ms))
+
     // Open position
     const openResult = await openPosition({
       symbol: driftSymbol,
@@ -306,6 +309,9 @@ export async function POST(request: NextRequest): Promise<NextResponse<ExecuteTr
       sizeUSD: positionSizeUSD,
       slippageTolerance: config.slippageTolerance,
     })
+    
+    // Wait 2 seconds before placing exit orders to space out RPC calls
+    await rpcDelay(2000)
 
     if (!openResult.success) {
       return NextResponse.json(
@@ -331,6 +337,9 @@ export async function POST(request: NextRequest): Promise<NextResponse<ExecuteTr
       
       try {
         console.log(`⚠️ Closing phantom position immediately for safety...`)
+        // Wait 2 seconds to space out RPC calls
+        await rpcDelay(2000)
+        
         closeResult = await closePosition({
           symbol: driftSymbol,
           percentToClose: 100, // Close 100% of whatever size exists