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-lock.json

@@ -11,6 +11,7 @@
       "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",
@@ -2863,6 +2864,21 @@
         "@babel/core": "^7.0.0"
       }
     },
+    "node_modules/bad-words": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/bad-words/-/bad-words-2.0.0.tgz",
+      "integrity": "sha512-NsfaHcGgNsPTlWl54HrXawEfsJ5TLcIVa5KQcMoGzQTycCECcYfcGRWwjuxdYz7PbgarFl9Epv/+qT5JV+oBtA==",
+      "license": "MIT",
+      "dependencies": {
+        "badwords-list": "^1.0.0"
+      }
+    },
+    "node_modules/badwords-list": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/badwords-list/-/badwords-list-1.0.0.tgz",
+      "integrity": "sha512-oWhaSG67e+HQj3OGHQt2ucP+vAPm1wTbdp2aDHeuh4xlGXBdWwzZ//pfu6swf5gZ8iX0b7JgmSo8BhgybbqszA==",
+      "license": "MIT"
+    },
     "node_modules/balanced-match": {
       "version": "1.0.2",
       "dev": true,