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/BookmarksViewModel.swift

@@ -88,6 +88,7 @@ final class BookmarksViewModel {
             restoreAIData(into: &all)
             bookmarks = try await claude.semanticSearch(query: searchQuery, in: all)
             nextPageUrl = nil
+            Analytics.track("search.semantic", ["query": searchQuery, "resultCount": bookmarks.count])
         } catch {
             self.error = error.localizedDescription
         }
@@ -159,6 +160,7 @@ final class BookmarksViewModel {
         do {
             let allResponse = try await api.fetchBookmarks(limit: 200)
             smartCollections = try await claude.generateCollections(from: allResponse.results)
+            Analytics.track("collections.generated", ["collectionCount": smartCollections.count])
         } catch {
             self.error = error.localizedDescription
         }
@@ -180,6 +182,8 @@ final class BookmarksViewModel {
                 break
             }
         }
+        let enriched = toEnrich.count - bookmarks.indices.filter { bookmarks[$0].aiSummary == nil }.count
+        if enriched > 0 { Analytics.track("enrich.completed", ["count": enriched]) }
         enrichmentProgress = 0
     }