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/Data/RR3DbContext.cs

@@ -19,6 +19,7 @@ public class RR3DbContext : DbContext
     public DbSet<CarUpgrade> CarUpgrades { get; set; }
     public DbSet<CareerProgress> CareerProgress { get; set; }
     public DbSet<GameAsset> GameAssets { get; set; }
+    public DbSet<ModPack> ModPacks { get; set; }
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -315,6 +316,14 @@ public class Car
     public int CashPrice { get; set; }
     public int GoldPrice { get; set; }
     public bool Available { get; set; } = true;
+    public int Year { get; set; }
+    public string? Description { get; set; }
+    
+    // Custom content fields
+    public bool IsCustom { get; set; }
+    public string? CustomAuthor { get; set; }
+    public string? CustomVersion { get; set; }
+    public DateTime? CreatedAt { get; set; }
 }
 
 public class OwnedCar
@@ -383,4 +392,32 @@ public class GameAsset
     public string? CarId { get; set; }
     public string? TrackId { get; set; }
     public string Category { get; set; } = "misc"; // models, textures, audio, etc.
+    public long? CompressedSize { get; set; }
+    public string? Md5Hash { get; set; }
+    
+    // Custom content support
+    public bool IsCustomContent { get; set; }
+    public string? CustomAuthor { get; set; }
+}
+
+// Mod Pack entity - bundles of custom content
+public class ModPack
+{
+    public int Id { get; set; }
+    public string PackId { get; set; } = string.Empty;
+    public string Name { get; set; } = string.Empty;
+    public string Author { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public string Version { get; set; } = "1.0";
+    
+    // Comma-separated IDs
+    public string? CarIds { get; set; }
+    public string? TrackIds { get; set; }
+    
+    // Statistics
+    public int DownloadCount { get; set; }
+    public double Rating { get; set; }
+    
+    public DateTime CreatedAt { get; set; }
+    public DateTime? UpdatedAt { get; set; }
 }