refactor: extract subsonic controller logic into specialized services

- Extract SubsonicRequestParser for HTTP parameter extraction
- Extract SubsonicResponseBuilder for XML/JSON response formatting
- Extract SubsonicModelMapper for search result parsing and merging
- Extract SubsonicProxyService for upstream Subsonic server communication
- Add comprehensive test coverage (45 tests) for all new services
- Reduce SubsonicController from 1174 to 666 lines (-43%)

All tests passing. Build succeeds with 0 errors.

octo-fiesta/Services/Subsonic/SubsonicProxyService.cs

@@ -0,0 +1,100 @@
+using Microsoft.AspNetCore.Mvc;
+using octo_fiesta.Models.Settings;
+
+namespace octo_fiesta.Services.Subsonic;
+
+/// <summary>
+/// Handles proxying requests to the underlying Subsonic server.
+/// </summary>
+public class SubsonicProxyService
+{
+    private readonly HttpClient _httpClient;
+    private readonly SubsonicSettings _subsonicSettings;
+
+    public SubsonicProxyService(
+        IHttpClientFactory httpClientFactory,
+        Microsoft.Extensions.Options.IOptions<SubsonicSettings> subsonicSettings)
+    {
+        _httpClient = httpClientFactory.CreateClient();
+        _subsonicSettings = subsonicSettings.Value;
+    }
+
+    /// <summary>
+    /// Relays a request to the Subsonic server and returns the response.
+    /// </summary>
+    public async Task<(byte[] Body, string? ContentType)> RelayAsync(
+        string endpoint, 
+        Dictionary<string, string> parameters)
+    {
+        var query = string.Join("&", parameters.Select(kv => 
+            $"{Uri.EscapeDataString(kv.Key)}={Uri.EscapeDataString(kv.Value)}"));
+        var url = $"{_subsonicSettings.Url}/{endpoint}?{query}";
+        
+        HttpResponseMessage response = await _httpClient.GetAsync(url);
+        response.EnsureSuccessStatusCode();
+        
+        var body = await response.Content.ReadAsByteArrayAsync();
+        var contentType = response.Content.Headers.ContentType?.ToString();
+        
+        return (body, contentType);
+    }
+
+    /// <summary>
+    /// Safely relays a request to the Subsonic server, returning null on failure.
+    /// </summary>
+    public async Task<(byte[]? Body, string? ContentType, bool Success)> RelaySafeAsync(
+        string endpoint, 
+        Dictionary<string, string> parameters)
+    {
+        try
+        {
+            var result = await RelayAsync(endpoint, parameters);
+            return (result.Body, result.ContentType, true);
+        }
+        catch
+        {
+            return (null, null, false);
+        }
+    }
+
+    /// <summary>
+    /// Relays a stream request to the Subsonic server with range processing support.
+    /// </summary>
+    public async Task<IActionResult> RelayStreamAsync(
+        Dictionary<string, string> parameters,
+        CancellationToken cancellationToken)
+    {
+        try
+        {
+            var query = string.Join("&", parameters.Select(kv => 
+                $"{Uri.EscapeDataString(kv.Key)}={Uri.EscapeDataString(kv.Value)}"));
+            var url = $"{_subsonicSettings.Url}/rest/stream?{query}";
+            
+            using var request = new HttpRequestMessage(HttpMethod.Get, url);
+            var response = await _httpClient.SendAsync(
+                request, 
+                HttpCompletionOption.ResponseHeadersRead, 
+                cancellationToken);
+            
+            if (!response.IsSuccessStatusCode)
+            {
+                return new StatusCodeResult((int)response.StatusCode);
+            }
+
+            var stream = await response.Content.ReadAsStreamAsync(cancellationToken);
+            var contentType = response.Content.Headers.ContentType?.ToString() ?? "audio/mpeg";
+            
+            return new FileStreamResult(stream, contentType)
+            {
+                EnableRangeProcessing = true
+            };
+        }
+        catch (Exception ex)
+        {
+            return new ObjectResult(new { error = $"Error streaming from Subsonic: {ex.Message}" })
+            {
+                StatusCode = 500
+            };
+        }
+    }
+}