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/SubsonicRequestParser.cs

@@ -0,0 +1,105 @@
+using Microsoft.AspNetCore.WebUtilities;
+using System.Text.Json;
+
+namespace octo_fiesta.Services.Subsonic;
+
+/// <summary>
+/// Service responsible for parsing HTTP request parameters from various sources
+/// (query string, form body, JSON body) for Subsonic API requests.
+/// </summary>
+public class SubsonicRequestParser
+{
+    /// <summary>
+    /// Extracts all parameters from an HTTP request (query parameters + body parameters).
+    /// Supports multiple content types: application/x-www-form-urlencoded and application/json.
+    /// </summary>
+    /// <param name="request">The HTTP request to parse</param>
+    /// <returns>Dictionary containing all extracted parameters</returns>
+    public async Task<Dictionary<string, string>> ExtractAllParametersAsync(HttpRequest request)
+    {
+        var parameters = new Dictionary<string, string>();
+
+        // Get query parameters
+        foreach (var query in request.Query)
+        {
+            parameters[query.Key] = query.Value.ToString();
+        }
+
+        // Get body parameters
+        if (request.ContentLength > 0 || request.ContentType != null)
+        {
+            // Handle application/x-www-form-urlencoded (OpenSubsonic formPost extension)
+            if (request.HasFormContentType)
+            {
+                await ExtractFormParametersAsync(request, parameters);
+            }
+            // Handle application/json
+            else if (request.ContentType?.Contains("application/json") == true)
+            {
+                await ExtractJsonParametersAsync(request, parameters);
+            }
+        }
+
+        return parameters;
+    }
+
+    /// <summary>
+    /// Extracts parameters from form-encoded request body.
+    /// </summary>
+    private async Task ExtractFormParametersAsync(HttpRequest request, Dictionary<string, string> parameters)
+    {
+        try
+        {
+            var form = await request.ReadFormAsync();
+            foreach (var field in form)
+            {
+                parameters[field.Key] = field.Value.ToString();
+            }
+        }
+        catch
+        {
+            // Fall back to manual parsing if ReadFormAsync fails
+            request.EnableBuffering();
+            using var reader = new StreamReader(request.Body, leaveOpen: true);
+            var body = await reader.ReadToEndAsync();
+            request.Body.Position = 0;
+            
+            if (!string.IsNullOrEmpty(body))
+            {
+                var formParams = QueryHelpers.ParseQuery(body);
+                foreach (var param in formParams)
+                {
+                    parameters[param.Key] = param.Value.ToString();
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Extracts parameters from JSON request body.
+    /// </summary>
+    private async Task ExtractJsonParametersAsync(HttpRequest request, Dictionary<string, string> parameters)
+    {
+        using var reader = new StreamReader(request.Body);
+        var body = await reader.ReadToEndAsync();
+        
+        if (!string.IsNullOrEmpty(body))
+        {
+            try
+            {
+                var bodyParams = JsonSerializer.Deserialize<Dictionary<string, object>>(body);
+                if (bodyParams != null)
+                {
+                    foreach (var param in bodyParams)
+                    {
+                        parameters[param.Key] = param.Value?.ToString() ?? "";
+                    }
+                }
+            }
+            catch (JsonException)
+            {
+                // Ignore JSON parsing errors
+            }
+        }
+    }
+}