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/schema.prisma

@@ -0,0 +1,128 @@
+// Prisma schema for spotlight.cam
+// Database: PostgreSQL 15
+
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+// Users table
+model User {
+  id            Int       @id @default(autoincrement())
+  username      String    @unique @db.VarChar(50)
+  email         String    @unique @db.VarChar(255)
+  passwordHash  String    @map("password_hash") @db.VarChar(255)
+  avatar        String?   @db.VarChar(255)
+  createdAt     DateTime  @default(now()) @map("created_at")
+  updatedAt     DateTime  @updatedAt @map("updated_at")
+
+  // Relations
+  messages      Message[]
+  matchesAsUser1 Match[]  @relation("MatchUser1")
+  matchesAsUser2 Match[]  @relation("MatchUser2")
+  ratingsGiven  Rating[]  @relation("RaterRatings")
+  ratingsReceived Rating[] @relation("RatedRatings")
+
+  @@map("users")
+}
+
+// Events table (dance events from worldsdc.com)
+model Event {
+  id                Int       @id @default(autoincrement())
+  name              String    @db.VarChar(255)
+  location          String    @db.VarChar(255)
+  startDate         DateTime  @map("start_date") @db.Date
+  endDate           DateTime  @map("end_date") @db.Date
+  worldsdcId        String?   @unique @map("worldsdc_id") @db.VarChar(100)
+  participantsCount Int       @default(0) @map("participants_count")
+  description       String?   @db.Text
+  createdAt         DateTime  @default(now()) @map("created_at")
+
+  // Relations
+  chatRooms         ChatRoom[]
+  matches           Match[]
+
+  @@map("events")
+}
+
+// Chat rooms (event chat and private 1:1 chat)
+model ChatRoom {
+  id        Int       @id @default(autoincrement())
+  eventId   Int?      @map("event_id")
+  type      String    @db.VarChar(20) // 'event' or 'private'
+  createdAt DateTime  @default(now()) @map("created_at")
+
+  // Relations
+  event     Event?    @relation(fields: [eventId], references: [id])
+  messages  Message[]
+  matches   Match[]
+
+  @@map("chat_rooms")
+}
+
+// Messages (text messages and video links)
+model Message {
+  id        Int       @id @default(autoincrement())
+  roomId    Int       @map("room_id")
+  userId    Int       @map("user_id")
+  content   String    @db.Text
+  type      String    @db.VarChar(20) // 'text', 'link', 'video'
+  createdAt DateTime  @default(now()) @map("created_at")
+
+  // Relations
+  room      ChatRoom  @relation(fields: [roomId], references: [id], onDelete: Cascade)
+  user      User      @relation(fields: [userId], references: [id])
+
+  @@index([roomId])
+  @@index([createdAt])
+  @@map("messages")
+}
+
+// Matches (pairs of users for collaboration)
+model Match {
+  id        Int       @id @default(autoincrement())
+  user1Id   Int       @map("user1_id")
+  user2Id   Int       @map("user2_id")
+  eventId   Int       @map("event_id")
+  roomId    Int?      @map("room_id")
+  status    String    @default("pending") @db.VarChar(20) // 'pending', 'accepted', 'completed'
+  createdAt DateTime  @default(now()) @map("created_at")
+
+  // Relations
+  user1     User      @relation("MatchUser1", fields: [user1Id], references: [id])
+  user2     User      @relation("MatchUser2", fields: [user2Id], references: [id])
+  event     Event     @relation(fields: [eventId], references: [id])
+  room      ChatRoom? @relation(fields: [roomId], references: [id])
+  ratings   Rating[]
+
+  @@unique([user1Id, user2Id, eventId])
+  @@index([user1Id])
+  @@index([user2Id])
+  @@index([eventId])
+  @@map("matches")
+}
+
+// Ratings (user ratings after collaboration)
+model Rating {
+  id                    Int       @id @default(autoincrement())
+  matchId               Int       @map("match_id")
+  raterId               Int       @map("rater_id")
+  ratedId               Int       @map("rated_id")
+  score                 Int       // 1-5
+  comment               String?   @db.Text
+  wouldCollaborateAgain Boolean   @default(false) @map("would_collaborate_again")
+  createdAt             DateTime  @default(now()) @map("created_at")
+
+  // Relations
+  match                 Match     @relation(fields: [matchId], references: [id])
+  rater                 User      @relation("RaterRatings", fields: [raterId], references: [id])
+  rated                 User      @relation("RatedRatings", fields: [ratedId], references: [id])
+
+  @@unique([matchId, raterId, ratedId])
+  @@index([ratedId])
+  @@map("ratings")
+}