test: fix backend test cleanup to preserve production data

Replace deleteMany({}) with selective cleanup targeting only test data:

- events.test.js: Delete only test users (john_dancer, sarah_swings, mike_blues)
  and test events (test-dance-festival-2025) before creating new ones
- matches.test.js: Clean up john_dancer, sarah_swings, mike_moves and
  test-dance-festival slug specifically
- users.test.js: Remove only john_dancer and sarah_swings test users
  in both beforeAll and afterAll hooks
- auth.test.js: Target specific test usernames/emails (testuser, newuser,
  lockouttest, etc.) instead of all users
- auth-phase1.5.test.js: Clean up 12 specific test users by username/email
- socket.test.js: Add beforeAll cleanup for sockettest user to prevent
  conflicts from previous test runs
- socket-webrtc.test.js: Clean up webrtc_user1 and webrtc_user2 before
  creating them

Fix CORS configuration for tests:
- security.js: Add http://localhost:3000 to allowed origins in development
  mode to fix app.test.js CORS test (was failing with 500 error)

Results: Improved from 125/223 passing to 137/223 passing (12 more tests fixed)
All test data cleanup now uses WHERE clauses with specific usernames/emails/slugs
instead of wiping entire tables with deleteMany({})

backend/src/__tests__/socket.test.js

@@ -15,6 +15,16 @@ describe('Socket.IO Server', () => {
   const port = 3001;
 
   beforeAll(async () => {
+    // Clean up existing test user if any
+    await prisma.user.deleteMany({
+      where: {
+        OR: [
+          { username: 'sockettest' },
+          { email: 'sockettest@example.com' }
+        ]
+      }
+    });
+
     // Create test user
     testUser = await prisma.user.create({
       data: {