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/app.js

@@ -27,10 +27,10 @@ app.get('/api/health', (req, res) => {
   });
 });
 
-// API routes (future)
+// API routes
+app.use('/api/events', require('./routes/events'));
 // app.use('/api/auth', require('./routes/auth'));
 // app.use('/api/users', require('./routes/users'));
-// app.use('/api/events', require('./routes/events'));
 // app.use('/api/matches', require('./routes/matches'));
 // app.use('/api/ratings', require('./routes/ratings'));