feat: implement comprehensive database connection pooling optimization

🎯 SESSION POOLING PERFORMANCE BREAKTHROUGH!

✅ Critical Issues Fixed:
- Eliminated multiple PrismaClient instances across schedulers
- Fixed connection pool exhaustion risk in processing modules
- Implemented singleton pattern for all database connections
- Added graceful shutdown and connection cleanup

🚀 Enhanced Pooling Features:
- Dual-mode connection pooling (standard + enhanced)
- PostgreSQL native pooling with @prisma/adapter-pg
- Advanced connection monitoring and health checks
- Configurable pool limits and timeouts via environment variables
- Real-time connection statistics and metrics

📊 Performance Optimizations:
- Single shared connection pool across all schedulers
- Configurable connection limits (DATABASE_CONNECTION_LIMIT=20)
- Idle timeout management (DATABASE_POOL_TIMEOUT=10)
- Connection cycling and health validation
- Process termination signal handling

🛠️ New Infrastructure:
- lib/database-pool.ts - Advanced pooling configuration
- app/api/admin/database-health/route.ts - Connection monitoring
- Enhanced lib/prisma.ts with dual-mode support
- Comprehensive documentation in docs/database-connection-pooling.md
- Graceful shutdown handling in lib/schedulers.ts

🎛️ Environment Configuration:
- USE_ENHANCED_POOLING=true for production optimization
- DATABASE_CONNECTION_LIMIT for pool size control
- DATABASE_POOL_TIMEOUT for idle connection management
- Automatic enhanced pooling in production environments

📈 Expected Impact:
- Eliminates connection pool exhaustion under load
- Reduces memory footprint from idle connections
- Improves scheduler performance and reliability
- Enables better resource monitoring and debugging
- Supports horizontal scaling with proper connection management

Production-ready connection pooling with monitoring and health checks!

lib/schedulers.ts

@@ -1,5 +1,6 @@
-// Combined scheduler initialization
+// Combined scheduler initialization with graceful shutdown
 
+import { prisma } from "./prisma.js";
 import { startProcessingScheduler } from "./processingScheduler";
 import { startCsvImportScheduler } from "./scheduler";
 
@@ -16,4 +17,44 @@ export function initializeSchedulers() {
   startProcessingScheduler();
 
   console.log("All schedulers initialized successfully");
+
+  // Set up graceful shutdown for schedulers
+  setupGracefulShutdown();
+}
+
+/**
+ * Set up graceful shutdown handling for schedulers and database connections
+ */
+function setupGracefulShutdown() {
+  const shutdown = async (signal: string) => {
+    console.log(`\nReceived ${signal}. Starting graceful shutdown...`);
+
+    try {
+      // Disconnect from database
+      await prisma.$disconnect();
+      console.log("Database connections closed.");
+
+      // Exit the process
+      process.exit(0);
+    } catch (error) {
+      console.error("Error during shutdown:", error);
+      process.exit(1);
+    }
+  };
+
+  // Handle various termination signals
+  process.on("SIGTERM", () => shutdown("SIGTERM"));
+  process.on("SIGINT", () => shutdown("SIGINT"));
+  process.on("SIGUSR2", () => shutdown("SIGUSR2")); // Nodemon restart
+
+  // Handle uncaught exceptions
+  process.on("uncaughtException", (error) => {
+    console.error("Uncaught Exception:", error);
+    shutdown("uncaughtException");
+  });
+
+  process.on("unhandledRejection", (reason, promise) => {
+    console.error("Unhandled Rejection at:", promise, "reason:", reason);
+    shutdown("unhandledRejection");
+  });
 }