Add podcast index: JSON mapping of article URLs to cached MP3 files

PodcastIndex.swift stores {articleUrl, filename, title, createdAt} in
App Support/podcasts/index.json. Updated on every successful download.
Enables building a podcast library view later.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Marks/Services/ClaudeService.swift

@@ -67,12 +67,13 @@ struct ClaudeService: Sendable {
         return try JSONDecoder().decode(PodcastStatus.self, from: data)
     }
 
-    func downloadPodcastAudio(jobId: String, articleUrl: String) async throws -> URL {
+    func downloadPodcastAudio(jobId: String, articleUrl: String, title: String? = nil) async throws -> URL {
         let dest = ClaudeService.cachedPodcastURL(for: articleUrl)
         try FileManager.default.createDirectory(at: dest.deletingLastPathComponent(),
                                                 withIntermediateDirectories: true)
         let data = try await get("/v1/podcast/audio/\(jobId)")
         try data.write(to: dest, options: .atomic)
+        PodcastIndex.upsert(articleUrl: articleUrl, filename: dest.lastPathComponent, title: title)
         return dest
     }