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/prisma/seed.js

@@ -0,0 +1,86 @@
+const { PrismaClient } = require('@prisma/client');
+
+const prisma = new PrismaClient();
+
+async function main() {
+  console.log('🌱 Seeding database...');
+
+  // Create events
+  const events = await Promise.all([
+    prisma.event.create({
+      data: {
+        name: 'Warsaw Dance Festival 2025',
+        location: 'Warsaw, Poland',
+        startDate: new Date('2025-03-15'),
+        endDate: new Date('2025-03-17'),
+        worldsdcId: 'wdf-2025',
+        participantsCount: 156,
+        description: 'The biggest West Coast Swing event in Central Europe',
+      },
+    }),
+    prisma.event.create({
+      data: {
+        name: 'Swing Camp Barcelona 2025',
+        location: 'Barcelona, Spain',
+        startDate: new Date('2025-04-20'),
+        endDate: new Date('2025-04-23'),
+        worldsdcId: 'scb-2025',
+        participantsCount: 203,
+        description: 'International swing dance camp with workshops and socials',
+      },
+    }),
+    prisma.event.create({
+      data: {
+        name: 'Blues Week Herräng 2025',
+        location: 'Herräng, Sweden',
+        startDate: new Date('2025-07-14'),
+        endDate: new Date('2025-07-20'),
+        worldsdcId: 'bwh-2025',
+        participantsCount: 89,
+        description: 'Week-long blues dance intensive in the heart of Sweden',
+      },
+    }),
+    prisma.event.create({
+      data: {
+        name: 'Krakow Swing Connection 2025',
+        location: 'Krakow, Poland',
+        startDate: new Date('2025-05-10'),
+        endDate: new Date('2025-05-12'),
+        worldsdcId: 'ksc-2025',
+        participantsCount: 127,
+        description: 'Three days of swing dancing in historic Krakow',
+      },
+    }),
+  ]);
+
+  console.log(`✅ Created ${events.length} events`);
+
+  // Create event chat rooms for each event
+  const chatRooms = await Promise.all(
+    events.map((event) =>
+      prisma.chatRoom.create({
+        data: {
+          eventId: event.id,
+          type: 'event',
+        },
+      })
+    )
+  );
+
+  console.log(`✅ Created ${chatRooms.length} event chat rooms`);
+
+  console.log('🎉 Seeding completed successfully!');
+  console.log('');
+  console.log('Created:');
+  console.log(`  - ${events.length} events`);
+  console.log(`  - ${chatRooms.length} chat rooms`);
+}
+
+main()
+  .catch((e) => {
+    console.error('❌ Seeding failed:', e);
+    process.exit(1);
+  })
+  .finally(async () => {
+    await prisma.$disconnect();
+  });