feat: add Chromecast playback support

Sources/App/ExternalSubtitles.swift

@@ -24,6 +24,13 @@ struct WebVTTDocument: Sendable {
     }
 
     static func load(url: String, headers: String) async throws -> WebVTTDocument {
+        let data = try await loadData(url: url, headers: headers)
+        // Providers sometimes disguise VTT resources as images: trust the explicit format and content.
+        guard let text = String(data: data, encoding: .utf8) else { throw SubtitleError.content }
+        return try parse(text)
+    }
+
+    static func loadData(url: String, headers: String) async throws -> Data {
         let request = try request(url: url, headers: headers)
         let configuration = URLSessionConfiguration.ephemeral
         configuration.timeoutIntervalForRequest = 15
@@ -51,10 +58,7 @@ struct WebVTTDocument: Sendable {
             if Task.isCancelled { throw CancellationError() }
             throw SubtitleError.transport
         }
-        // Providers sometimes disguise VTT resources as images: trust the explicit format and content,
-        // never the path extension or Content-Type.
-        guard let text = String(data: data, encoding: .utf8) else { throw SubtitleError.content }
-        return try parse(text)
+        return data
     }
 
     static func parse(_ text: String) throws -> WebVTTDocument {