feat: add test accounts and secure event slugs

Security improvements:
- Add @default(cuid()) to Event.slug for auto-generated random slugs
- Prevent ID enumeration attacks (no more predictable slugs like "warsaw-dance-festival-2025")
- Event slugs now generated as secure cuid strings (e.g., "cmhz3lcgb00018vbn34v4phoi")

Test accounts:
- Add 3 test users to seed (john_dancer, sarah_swings, mike_blues)
- All users checked in to Warsaw Dance Festival 2025
- Pre-configured heats for testing matchmaking system
- Full profiles with WSDC IDs, social media, and locations

Seed improvements:
- Add bcryptjs for password hashing
- Add Prisma seed configuration to package.json
- Use worldsdcId for event upsert (instead of slug)
- Auto-generate event slugs via Prisma default

Documentation:
- Add test account credentials to SESSION_CONTEXT.md
- Document event slug security model
- Include sample heats for each test user

backend/prisma/schema.prisma

@@ -62,7 +62,7 @@ model User {
 // Events table (dance events from worldsdc.com)
 model Event {
   id                Int       @id @default(autoincrement())
-  slug              String    @unique @db.VarChar(50)
+  slug              String    @unique @default(cuid()) @db.VarChar(50)
   name              String    @db.VarChar(255)
   location          String    @db.VarChar(255)
   startDate         DateTime  @map("start_date") @db.Date