Add iOS Langfuse analytics via backend events endpoint

AnalyticsService.track() is fire-and-forget — posts to POST /v1/marks/events
which records a Langfuse trace. No external SDK, no Sentry on iOS side.

Events tracked:
- podcast.started (url, cached)
- podcast.completed (url, durationSec) — fires at 97% playback
- podcast.failed (url, error)
- search.semantic (query, resultCount)
- enrich.completed (count)
- collections.generated (collectionCount)

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

Marks/Views/PodcastPlayerView.swift

@@ -59,10 +59,12 @@ final class PodcastPlayerViewModel {
 
         let cached = ClaudeService.cachedPodcastURL(for: articleUrl)
         if FileManager.default.fileExists(atPath: cached.path) {
+            Analytics.track("podcast.started", ["url": articleUrl, "cached": true])
             setupPlayer(url: cached, title: "Podcast")
             return
         }
 
+        Analytics.track("podcast.started", ["url": articleUrl, "cached": false])
         phase = .generating(progress: 0, label: "Starting…")
 
         pollingTask = Task {
@@ -81,13 +83,16 @@ final class PodcastPlayerViewModel {
                         return
                     }
                     if job.isFailed {
-                        phase = .failed(job.error ?? "Generation failed")
+                        let errMsg = job.error ?? "Generation failed"
+                        Analytics.track("podcast.failed", ["url": articleUrl, "error": errMsg])
+                        phase = .failed(errMsg)
                         return
                     }
                     try await Task.sleep(for: .seconds(3))
                 }
             } catch is CancellationError {
             } catch {
+                Analytics.track("podcast.failed", ["url": articleUrl, "error": error.localizedDescription])
                 phase = .failed(error.localizedDescription)
             }
         }
@@ -155,8 +160,14 @@ final class PodcastPlayerViewModel {
 
         let interval = CMTime(seconds: 0.5, preferredTimescale: 600)
         timeObserver = p.addPeriodicTimeObserver(forInterval: interval, queue: .main) { [weak self] t in
-            self?.currentTime = t.seconds
-            self?.updateNowPlaying()
+            guard let self else { return }
+            let prev = self.currentTime
+            self.currentTime = t.seconds
+            self.updateNowPlaying()
+            // Track completion when crossing 97%
+            if self.duration > 1 && prev / self.duration < 0.97 && t.seconds / self.duration >= 0.97 {
+                Analytics.track("podcast.completed", ["url": self.currentArticleUrl, "durationSec": Int(self.duration)])
+            }
         }
 
         setupRemoteCommands()