feat: Helius primary + Alchemy fallback for trade execution

- Helius HTTPS: Primary RPC for Drift SDK initialization and subscriptions
- Alchemy HTTPS (10K CU/s): Fallback RPC for transaction confirmations
- Added getFallbackConnection() method to DriftService
- openPosition() and closePosition() now use Alchemy for tx confirmations
- accountSubscribe errors are non-fatal warnings (SDK falls back gracefully)
- System fully operational: Drift initialized, Position Manager ready
- Trade execution will use high-throughput Alchemy for confirmations

lib/drift/client.ts

@@ -470,6 +470,13 @@ export class DriftService {
     return this.connection
   }
 
+  /**
+   * Get fallback connection (Alchemy for trade execution)
+   */
+  getFallbackConnection(): Connection | null {
+    return this.fallbackConnection || null
+  }
+
   /**
    * Get user instance
    */