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-lock.json

@@ -9,6 +9,7 @@
       "version": "1.0.0",
       "license": "ISC",
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         "cors": "^2.8.5",
         "dotenv": "^16.3.1",
         "express": "^4.18.2"
@@ -16,6 +17,7 @@
       "devDependencies": {
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         "nodemon": "^3.0.2",
+        "prisma": "^5.8.0",
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       }
     },
@@ -977,6 +979,74 @@
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+      "devOptional": true,
+      "hasInstallScript": true,
+      "license": "Apache-2.0",
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+      "resolved": "https://registry.npmjs.org/@prisma/engines-version/-/engines-version-5.22.0-44.605197351a3c8bdd595af2d2a9bc3025bca48ea2.tgz",
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+      "devOptional": true,
+      "license": "Apache-2.0"
+    },
+    "node_modules/@prisma/fetch-engine": {
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+      "resolved": "https://registry.npmjs.org/@prisma/fetch-engine/-/fetch-engine-5.22.0.tgz",
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+      "devOptional": true,
+      "license": "Apache-2.0",
+      "dependencies": {
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+        "@prisma/engines-version": "5.22.0-44.605197351a3c8bdd595af2d2a9bc3025bca48ea2",
+        "@prisma/get-platform": "5.22.0"
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+    "node_modules/@prisma/get-platform": {
+      "version": "5.22.0",
+      "resolved": "https://registry.npmjs.org/@prisma/get-platform/-/get-platform-5.22.0.tgz",
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     "node_modules/@sinclair/typebox": {
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@@ -3930,6 +4000,26 @@
         "url": "https://github.com/chalk/ansi-styles?sponsor=1"
       }
     },
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+      "hasInstallScript": true,
+      "license": "Apache-2.0",
+      "dependencies": {
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+      "engines": {
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