Add asset management system

- Created Assets.cshtml and Assets.cshtml.cs for admin panel
- Upload assets with MD5/SHA256 hash calculation
- Generate asset manifests in RR3 format (tab-separated)
- Integrated with Nimble SDK asset download system
- Updated GameAsset model with IsRequired, UploadedAt, Description
- Added navigation link in _Layout.cshtml
- Supports categories: base, cars, tracks, audio, textures, UI, DLC
- Asset download endpoint at /content/api/{assetPath}
- Manifest endpoint at /content/api/manifest

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

RR3CommunityServer/Data/RR3DbContext.cs

@@ -381,16 +381,18 @@ public class GameAsset
     public string? EaCdnPath { get; set; }
     
     // Local storage
-    public string LocalPath { get; set; } = string.Empty;
+    public string? LocalPath { get; set; }
     public long FileSize { get; set; }
-    public string FileSha256 { get; set; } = string.Empty;
+    public string? FileSha256 { get; set; }
     public string? Version { get; set; }
     
     // Metadata
     public DateTime DownloadedAt { get; set; } = DateTime.UtcNow;
+    public DateTime UploadedAt { get; set; } = DateTime.UtcNow;
     public DateTime LastAccessedAt { get; set; } = DateTime.UtcNow;
     public int AccessCount { get; set; } = 0;
     public bool IsAvailable { get; set; } = true;
+    public bool IsRequired { get; set; } = false;
     
     // Game-specific (optional)
     public string? CarId { get; set; }
@@ -398,6 +400,7 @@ public class GameAsset
     public string Category { get; set; } = "misc"; // models, textures, audio, etc.
     public long? CompressedSize { get; set; }
     public string? Md5Hash { get; set; }
+    public string? Description { get; set; }
     
     // Custom content support
     public bool IsCustomContent { get; set; }