feat: Hybrid RPC fallback system (Alchemy → Helius)

- Automatic fallback after 2 consecutive rate limits
- Primary: Alchemy (300M CU/month, stable for normal ops)
- Fallback: Helius (10 req/sec, backup for startup bursts)
- Reduced startup validation: 6h window, 5 trades (was 24h, 20 trades)
- Multi-position safety check (prevents order cancellation conflicts)
- Rate limit-aware retry logic with exponential backoff

Implementation:
- lib/drift/client.ts: Added fallbackConnection, switchToFallbackRpc()
- .env: SOLANA_FALLBACK_RPC_URL configuration
- lib/startup/init-position-manager.ts: Reduced validation scope
- lib/trading/position-manager.ts: Multi-position order protection

Tested: System switched to fallback on startup, Position Manager active
Result: 1 active trade being monitored after automatic RPC switch

lib/startup/init-position-manager.ts

@@ -52,6 +52,7 @@ async function validateOpenTrades() {
     
     // Get both truly open trades AND recently "closed" trades (last 24h)
     // Recently closed trades might still be open if close transaction failed
+    // TEMPORARILY REDUCED: Check only last 5 closed trades to avoid rate limiting on startup
     const [openTrades, recentlyClosedTrades] = await Promise.all([
       prisma.trade.findMany({
         where: { status: 'open' },
@@ -60,10 +61,10 @@ async function validateOpenTrades() {
       prisma.trade.findMany({
         where: {
           exitReason: { not: null },
-          exitTime: { gte: new Date(Date.now() - 24 * 60 * 60 * 1000) } // Last 24 hours
+          exitTime: { gte: new Date(Date.now() - 6 * 60 * 60 * 1000) } // Last 6 hours (reduced from 24h)
         },
         orderBy: { exitTime: 'desc' },
-        take: 20 // Check last 20 closed trades
+        take: 5 // Reduced from 20 to avoid rate limiting
       })
     ])