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/PodcastIndex.swift

@@ -0,0 +1,56 @@
+import Foundation
+
+struct PodcastEntry: Codable, Identifiable {
+    var id: String { articleUrl }
+    let articleUrl: String
+    let filename: String
+    var title: String?
+    let createdAt: Date
+}
+
+enum PodcastIndex {
+    private static let indexURL: URL = {
+        let dir = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask)[0]
+            .appendingPathComponent("podcasts")
+        return dir.appendingPathComponent("index.json")
+    }()
+
+    private static let encoder: JSONEncoder = {
+        let e = JSONEncoder()
+        e.dateEncodingStrategy = .iso8601
+        e.outputFormatting = .prettyPrinted
+        return e
+    }()
+
+    private static let decoder: JSONDecoder = {
+        let d = JSONDecoder()
+        d.dateDecodingStrategy = .iso8601
+        return d
+    }()
+
+    static func all() -> [PodcastEntry] {
+        guard let data = try? Data(contentsOf: indexURL),
+              let entries = try? decoder.decode([PodcastEntry].self, from: data)
+        else { return [] }
+        return entries.sorted { $0.createdAt > $1.createdAt }
+    }
+
+    static func upsert(articleUrl: String, filename: String, title: String?) {
+        var entries = all()
+        entries.removeAll { $0.articleUrl == articleUrl }
+        entries.insert(PodcastEntry(articleUrl: articleUrl, filename: filename,
+                                    title: title, createdAt: Date()), at: 0)
+        save(entries)
+    }
+
+    static func remove(articleUrl: String) {
+        var entries = all()
+        entries.removeAll { $0.articleUrl == articleUrl }
+        save(entries)
+    }
+
+    private static func save(_ entries: [PodcastEntry]) {
+        guard let data = try? encoder.encode(entries) else { return }
+        try? data.write(to: indexURL, options: .atomic)
+    }
+}