feat: support YouTube VTT subtitle selection

Sources/App/ResolvedSource.swift

@@ -7,6 +7,8 @@ struct ResolvedSubtitle: Decodable, Sendable {
     let language: String
     let label: String
     let isDefault: Bool
+    // Extractor identity survives expiring URLs and distinguishes manual from automatic captions.
+    var selectionID: String? = nil
     private let headerFields: [String: String]
     var proxy: MiruroProxy? = nil
     var headers: String {
@@ -15,7 +17,7 @@ struct ResolvedSubtitle: Decodable, Sendable {
     fileprivate var validatedHeaders: [String: String] { headerFields }
 
     init(url: String, format: String, language: String, label: String, isDefault: Bool,
-         headers: [String: String], proxy: MiruroProxy? = nil) {
+         headers: [String: String], proxy: MiruroProxy? = nil, selectionID: String? = nil) {
         self.url = url
         self.format = format
         self.language = language
@@ -23,6 +25,7 @@ struct ResolvedSubtitle: Decodable, Sendable {
         self.isDefault = isDefault
         self.headerFields = headers
         self.proxy = proxy
+        self.selectionID = selectionID
     }
     private enum CodingKeys: String, CodingKey {
         case url, format, language, label