Add Game Asset Preservation System Foundation

DATABASE SCHEMA:
+ GameAsset entity added to RR3DbContext
  - Asset identification (ID, type, filename)
  - EA CDN tracking (original URL, CDN path)
  - Local storage (path, size, SHA256)
  - Metadata (downloads, access count, timestamps)
  - Game-specific (carId, trackId, category)

FEATURES:
- Track all cached assets in database
- Store EA CDN URLs while available
- SHA256 integrity checking
- Access statistics
- Category organization
- Version tracking

PURPOSE:
Foundation for full asset caching system.
Next step: AssetsController with EA CDN proxying.

STORAGE STRUCTURE:
wwwroot/assets/
├── cars/
├── tracks/
├── textures/
├── audio/
├── models/
└── misc/

PRESERVATION STRATEGY:
Phase 1 (EA online): Proxy + cache assets
Phase 2 (EA offline): Serve from cache
Result: Complete game preservation! 🎮💾

Package added: Microsoft.Extensions.Http (for CDN proxying)

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

RR3CommunityServer/Data/RR3DbContext.cs

@@ -18,6 +18,7 @@ public class RR3DbContext : DbContext
     public DbSet<OwnedCar> OwnedCars { get; set; }
     public DbSet<CarUpgrade> CarUpgrades { get; set; }
     public DbSet<CareerProgress> CareerProgress { get; set; }
+    public DbSet<GameAsset> GameAssets { get; set; }
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -351,3 +352,35 @@ public class CareerProgress
     public double BestTime { get; set; }
     public DateTime? CompletedAt { get; set; }
 }
+
+public class GameAsset
+{
+    public int Id { get; set; }
+    
+    // Asset identification
+    public string AssetId { get; set; } = string.Empty;
+    public string AssetType { get; set; } = string.Empty; // car, track, texture, audio, etc.
+    public string FileName { get; set; } = string.Empty;
+    public string ContentType { get; set; } = "application/octet-stream";
+    
+    // EA CDN info
+    public string? OriginalUrl { get; set; }
+    public string? EaCdnPath { get; set; }
+    
+    // Local storage
+    public string LocalPath { get; set; } = string.Empty;
+    public long FileSize { get; set; }
+    public string FileSha256 { get; set; } = string.Empty;
+    public string? Version { get; set; }
+    
+    // Metadata
+    public DateTime DownloadedAt { get; set; } = DateTime.UtcNow;
+    public DateTime LastAccessedAt { get; set; } = DateTime.UtcNow;
+    public int AccessCount { get; set; } = 0;
+    public bool IsAvailable { get; set; } = true;
+    
+    // Game-specific (optional)
+    public string? CarId { get; set; }
+    public string? TrackId { get; set; }
+    public string Category { get; set; } = "misc"; // models, textures, audio, etc.
+}