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/utils/db.js

@@ -0,0 +1,41 @@
+const { PrismaClient } = require('@prisma/client');
+
+// Singleton instance of Prisma Client
+let prisma;
+
+if (process.env.NODE_ENV === 'production') {
+  prisma = new PrismaClient();
+} else {
+  // In development, use a global variable to prevent multiple instances
+  // during hot reload
+  if (!global.prisma) {
+    global.prisma = new PrismaClient({
+      log: ['error', 'warn'],
+    });
+  }
+  prisma = global.prisma;
+}
+
+// Test database connection
+async function testConnection() {
+  try {
+    await prisma.$connect();
+    console.log('✅ Database connected successfully');
+    return true;
+  } catch (error) {
+    console.error('❌ Database connection failed:', error.message);
+    return false;
+  }
+}
+
+// Graceful shutdown
+async function disconnect() {
+  await prisma.$disconnect();
+  console.log('Database disconnected');
+}
+
+module.exports = {
+  prisma,
+  testConnection,
+  disconnect,
+};