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/package.json

@@ -7,7 +7,11 @@
     "start": "node src/server.js",
     "dev": "nodemon src/server.js",
     "test": "jest --coverage",
-    "test:watch": "jest --watch"
+    "test:watch": "jest --watch",
+    "prisma:generate": "prisma generate",
+    "prisma:migrate": "prisma migrate dev",
+    "prisma:seed": "node prisma/seed.js",
+    "prisma:studio": "prisma studio"
   },
   "keywords": ["webrtc", "p2p", "video", "dance", "matchmaking"],
   "author": "",
@@ -15,12 +19,14 @@
   "dependencies": {
     "express": "^4.18.2",
     "cors": "^2.8.5",
-    "dotenv": "^16.3.1"
+    "dotenv": "^16.3.1",
+    "@prisma/client": "^5.8.0"
   },
   "devDependencies": {
     "nodemon": "^3.0.2",
     "jest": "^29.7.0",
-    "supertest": "^6.3.3"
+    "supertest": "^6.3.3",
+    "prisma": "^5.8.0"
   },
   "jest": {
     "testEnvironment": "node",