feat: add PostgreSQL database with Prisma ORM

Phase 1 - Step 2: PostgreSQL Setup

**Infrastructure:**
- Add PostgreSQL 15 Alpine container to docker-compose.yml
- Configure persistent volume for database data
- Update backend Dockerfile with OpenSSL for Prisma compatibility

**Database Schema (Prisma):**
- 6 tables: users, events, chat_rooms, messages, matches, ratings
- Foreign key relationships and cascading deletes
- Performance indexes on frequently queried columns
- Unique constraints for data integrity

**Prisma Setup:**
- Prisma Client for database queries
- Migration system with initial migration
- Seed script with 4 test events and chat rooms
- Database connection utility with singleton pattern

**API Implementation:**
- GET /api/events - List all events (with filtering and sorting)
- GET /api/events/:id - Get single event with relations
- Database connection test on server startup
- Graceful database disconnect on shutdown

**Seed Data:**
- Warsaw Dance Festival 2025
- Swing Camp Barcelona 2025
- Blues Week Herräng 2025
- Krakow Swing Connection 2025

**Testing:**
- Database connection verified ✅
- API endpoints returning data from PostgreSQL ✅
- Migrations applied successfully ✅

All systems operational 🚀

backend/src/server.js

@@ -1,31 +1,40 @@
 require('dotenv').config();
 const app = require('./app');
+const { testConnection, disconnect } = require('./utils/db');
 
 const PORT = process.env.PORT || 3000;
 
-const server = app.listen(PORT, '0.0.0.0', () => {
-  console.log('=================================');
-  console.log('🚀 spotlight.cam Backend Started');
-  console.log('=================================');
-  console.log(`Environment: ${process.env.NODE_ENV || 'development'}`);
-  console.log(`Server running on port: ${PORT}`);
-  console.log(`Health check: http://localhost:${PORT}/api/health`);
-  console.log('=================================');
+async function startServer() {
+  // Test database connection
+  await testConnection();
+
+  const server = app.listen(PORT, '0.0.0.0', () => {
+    console.log('=================================');
+    console.log('🚀 spotlight.cam Backend Started');
+    console.log('=================================');
+    console.log(`Environment: ${process.env.NODE_ENV || 'development'}`);
+    console.log(`Server running on port: ${PORT}`);
+    console.log(`Health check: http://localhost:${PORT}/api/health`);
+    console.log('=================================');
+  });
+
+  return server;
+}
+
+startServer().catch((err) => {
+  console.error('Failed to start server:', err);
+  process.exit(1);
 });
 
 // Graceful shutdown
-process.on('SIGTERM', () => {
+process.on('SIGTERM', async () => {
   console.log('SIGTERM received, shutting down gracefully...');
-  server.close(() => {
-    console.log('Server closed');
-    process.exit(0);
-  });
+  await disconnect();
+  process.exit(0);
 });
 
-process.on('SIGINT', () => {
+process.on('SIGINT', async () => {
   console.log('SIGINT received, shutting down gracefully...');
-  server.close(() => {
-    console.log('Server closed');
-    process.exit(0);
-  });
+  await disconnect();
+  process.exit(0);
 });