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/package.json

@@ -54,5 +54,8 @@
     "testMatch": [
       "**/__tests__/**/*.test.js"
     ]
+  },
+  "prisma": {
+    "seed": "node prisma/seed.js"
   }
 }