Add full custom content & modding system 🎨

MASSIVE FEATURE: Turn RR3 into a moddable community platform!

Controllers:
- ModdingController: Upload/manage custom cars & tracks
  - POST /modding/api/cars/upload (custom car upload)
  - POST /modding/api/tracks/upload (custom track upload)
  - GET /modding/api/cars (list custom content)
  - GET /modding/api/content (search & filter)
  - POST /modding/api/modpack/create (bundle mods)
  - GET /modding/api/modpacks (browse packs)
  - DELETE /modding/api/content/{id} (moderation)

Database:
- Added ModPack entity for mod bundles
- Extended Car with IsCustom, CustomAuthor, CustomVersion
- Extended GameAsset with IsCustomContent, CustomAuthor
- Supports versioning, ratings, download tracking

Configuration:
- CustomAssetsPath & ModsPath settings
- EnableModding flag
- Upload size limits (100MB cars, 200MB tracks)

Documentation:
- MODDING_GUIDE.md: Complete modding system guide
  - API endpoints & examples
  - Content creation workflow
  - Tools & resources
  - Community guidelines
  - Example scripts

Features:
✅ Upload custom cars (models, textures, audio)
✅ Upload custom tracks (layouts, scenery)
✅ Create & share mod packs
✅ Version control & ratings
✅ Community content discovery
✅ Automatic MD5 verification
✅ Organized file storage

This makes RR3 a COMMUNITY-DRIVEN platform that can
live forever with user-generated content! 🎮🏎️

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

RR3CommunityServer/appsettings.json

@@ -7,11 +7,16 @@
   },
   "AllowedHosts": "*",
   "AssetsBasePath": "Assets/downloaded",
+  "CustomAssetsPath": "Assets/custom",
+  "ModsPath": "Assets/mods",
   "ServerSettings": {
     "AllowSelfSignedCerts": true,
     "EnableAssetDownloads": true,
     "FreeGoldPurchases": true,
     "UnlockAllCars": false,
-    "UnlimitedCurrency": false
+    "UnlimitedCurrency": false,
+    "EnableModding": true,
+    "MaxCustomCarUploadSizeMB": 100,
+    "MaxCustomTrackUploadSizeMB": 200
   }
 }