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/migrations/20251112205214_init/migration.sql

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+-- CreateTable
+CREATE TABLE "users" (
+    "id" SERIAL NOT NULL,
+    "username" VARCHAR(50) NOT NULL,
+    "email" VARCHAR(255) NOT NULL,
+    "password_hash" VARCHAR(255) NOT NULL,
+    "avatar" VARCHAR(255),
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "users_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "events" (
+    "id" SERIAL NOT NULL,
+    "name" VARCHAR(255) NOT NULL,
+    "location" VARCHAR(255) NOT NULL,
+    "start_date" DATE NOT NULL,
+    "end_date" DATE NOT NULL,
+    "worldsdc_id" VARCHAR(100),
+    "participants_count" INTEGER NOT NULL DEFAULT 0,
+    "description" TEXT,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "events_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "chat_rooms" (
+    "id" SERIAL NOT NULL,
+    "event_id" INTEGER,
+    "type" VARCHAR(20) NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "chat_rooms_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "messages" (
+    "id" SERIAL NOT NULL,
+    "room_id" INTEGER NOT NULL,
+    "user_id" INTEGER NOT NULL,
+    "content" TEXT NOT NULL,
+    "type" VARCHAR(20) NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "messages_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "matches" (
+    "id" SERIAL NOT NULL,
+    "user1_id" INTEGER NOT NULL,
+    "user2_id" INTEGER NOT NULL,
+    "event_id" INTEGER NOT NULL,
+    "room_id" INTEGER,
+    "status" VARCHAR(20) NOT NULL DEFAULT 'pending',
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "matches_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "ratings" (
+    "id" SERIAL NOT NULL,
+    "match_id" INTEGER NOT NULL,
+    "rater_id" INTEGER NOT NULL,
+    "rated_id" INTEGER NOT NULL,
+    "score" INTEGER NOT NULL,
+    "comment" TEXT,
+    "would_collaborate_again" BOOLEAN NOT NULL DEFAULT false,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "ratings_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "users_username_key" ON "users"("username");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "users_email_key" ON "users"("email");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "events_worldsdc_id_key" ON "events"("worldsdc_id");
+
+-- CreateIndex
+CREATE INDEX "messages_room_id_idx" ON "messages"("room_id");
+
+-- CreateIndex
+CREATE INDEX "messages_created_at_idx" ON "messages"("created_at");
+
+-- CreateIndex
+CREATE INDEX "matches_user1_id_idx" ON "matches"("user1_id");
+
+-- CreateIndex
+CREATE INDEX "matches_user2_id_idx" ON "matches"("user2_id");
+
+-- CreateIndex
+CREATE INDEX "matches_event_id_idx" ON "matches"("event_id");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "matches_user1_id_user2_id_event_id_key" ON "matches"("user1_id", "user2_id", "event_id");
+
+-- CreateIndex
+CREATE INDEX "ratings_rated_id_idx" ON "ratings"("rated_id");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "ratings_match_id_rater_id_rated_id_key" ON "ratings"("match_id", "rater_id", "rated_id");
+
+-- AddForeignKey
+ALTER TABLE "chat_rooms" ADD CONSTRAINT "chat_rooms_event_id_fkey" FOREIGN KEY ("event_id") REFERENCES "events"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "messages" ADD CONSTRAINT "messages_room_id_fkey" FOREIGN KEY ("room_id") REFERENCES "chat_rooms"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "messages" ADD CONSTRAINT "messages_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "users"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "matches" ADD CONSTRAINT "matches_user1_id_fkey" FOREIGN KEY ("user1_id") REFERENCES "users"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "matches" ADD CONSTRAINT "matches_user2_id_fkey" FOREIGN KEY ("user2_id") REFERENCES "users"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "matches" ADD CONSTRAINT "matches_event_id_fkey" FOREIGN KEY ("event_id") REFERENCES "events"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "matches" ADD CONSTRAINT "matches_room_id_fkey" FOREIGN KEY ("room_id") REFERENCES "chat_rooms"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ratings" ADD CONSTRAINT "ratings_match_id_fkey" FOREIGN KEY ("match_id") REFERENCES "matches"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ratings" ADD CONSTRAINT "ratings_rater_id_fkey" FOREIGN KEY ("rater_id") REFERENCES "users"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ratings" ADD CONSTRAINT "ratings_rated_id_fkey" FOREIGN KEY ("rated_id") REFERENCES "users"("id") ON DELETE RESTRICT ON UPDATE CASCADE;

backend/prisma/migrations/migration_lock.toml

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+# Please do not edit this file manually
+# It should be added in your version-control system (i.e. Git)
+provider = "postgresql"