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/routes/events.js

@@ -0,0 +1,71 @@
+const express = require('express');
+const { prisma } = require('../utils/db');
+
+const router = express.Router();
+
+// GET /api/events - List all events
+router.get('/', async (req, res, next) => {
+  try {
+    const events = await prisma.event.findMany({
+      orderBy: {
+        startDate: 'asc',
+      },
+      select: {
+        id: true,
+        name: true,
+        location: true,
+        startDate: true,
+        endDate: true,
+        worldsdcId: true,
+        participantsCount: true,
+        description: true,
+        createdAt: true,
+      },
+    });
+
+    res.json({
+      success: true,
+      count: events.length,
+      data: events,
+    });
+  } catch (error) {
+    next(error);
+  }
+});
+
+// GET /api/events/:id - Get event by ID
+router.get('/:id', async (req, res, next) => {
+  try {
+    const { id } = req.params;
+
+    const event = await prisma.event.findUnique({
+      where: {
+        id: parseInt(id),
+      },
+      include: {
+        chatRooms: true,
+        _count: {
+          select: {
+            matches: true,
+          },
+        },
+      },
+    });
+
+    if (!event) {
+      return res.status(404).json({
+        success: false,
+        error: 'Event not found',
+      });
+    }
+
+    res.json({
+      success: true,
+      data: event,
+    });
+  } catch (error) {
+    next(error);
+  }
+});
+
+module.exports = router;