Add MusicBrainz API integration for metadata enrichment

- Added MusicBrainzSettings model with username/password authentication
- Created MusicBrainzService with ISRC lookup and recording search
- Implements proper rate limiting (1 req/sec) per MusicBrainz rules
- Added meaningful User-Agent header as required
- Registered service in Program.cs with configuration
- Added MusicBrainz section to appsettings.json
- Credentials stored in .env (MUSICBRAINZ_USERNAME/PASSWORD)

Next: Add to admin UI and implement import/export for .env

allstarr/Models/Settings/MusicBrainzSettings.cs

@@ -0,0 +1,21 @@
+namespace allstarr.Models.Settings;
+
+/// <summary>
+/// Settings for MusicBrainz API integration.
+/// </summary>
+public class MusicBrainzSettings
+{
+    public bool Enabled { get; set; } = true;
+    public string? Username { get; set; }
+    public string? Password { get; set; }
+    
+    /// <summary>
+    /// Base URL for MusicBrainz API.
+    /// </summary>
+    public string BaseUrl { get; set; } = "https://musicbrainz.org/ws/2";
+    
+    /// <summary>
+    /// Rate limit: 1 request per second for unauthenticated, 1 per second for authenticated.
+    /// </summary>
+    public int RateLimitMs { get; set; } = 1000;
+}