feat(chat): implement spam protection and profanity filter

Add comprehensive message validation with three protection mechanisms:

1. Rate Limiting: 10 messages per minute per user
2. Duplicate Detection: Prevents sending identical messages within 1 minute
3. Profanity Filter: Blocks inappropriate language (English + Polish)

Implementation:
- New messageValidation.js middleware with validateMessage() function
- Integrated into both event chat and match chat handlers
- Uses bad-words library (v2.0.0 for CommonJS compatibility)
- In-memory tracking with automatic cleanup every 5 minutes
- User-friendly error messages for each validation type

Technical details:
- Rate limit: 10 msg/min sliding window
- Duplicate check: Last 5 messages within 60s window
- Profanity: bad-words + 11 Polish words
- Memory management: Periodic cleanup of expired data

backend/package.json

@@ -26,6 +26,7 @@
   "dependencies": {
     "@aws-sdk/client-ses": "^3.930.0",
     "@prisma/client": "^5.8.0",
+    "bad-words": "^2.0.0",
     "bcryptjs": "^2.4.3",
     "cookie-parser": "^1.4.7",
     "cors": "^2.8.5",
@@ -49,7 +50,9 @@
   },
   "jest": {
     "testEnvironment": "node",
-    "setupFilesAfterEnv": ["<rootDir>/jest.setup.js"],
+    "setupFilesAfterEnv": [
+      "<rootDir>/jest.setup.js"
+    ],
     "coveragePathIgnorePatterns": [
       "/node_modules/"
     ],