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/Services/AnalyticsService.swift

@@ -0,0 +1,26 @@
+import Foundation
+
+enum Analytics {
+    static func track(_ event: String, _ properties: [String: Any] = [:]) {
+        Task.detached {
+            do {
+                let token = try await MarksAuth.validToken()
+                guard let url = URL(string: MarksAuth.baseURL + "/v1/marks/events") else { return }
+                var request = URLRequest(url: url)
+                request.httpMethod = "POST"
+                request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
+                request.setValue("application/json", forHTTPHeaderField: "Content-Type")
+                var body: [String: Any] = ["event": event]
+                if !properties.isEmpty { body["properties"] = properties }
+                request.httpBody = try JSONSerialization.data(withJSONObject: body)
+                let (_, response) = try await URLSession.shared.data(for: request)
+                let status = (response as? HTTPURLResponse)?.statusCode ?? 0
+                if status < 200 || status > 299 {
+                    print("[Analytics] \(event) → HTTP \(status)")
+                }
+            } catch {
+                print("[Analytics] \(event) failed: \(error.localizedDescription)")
+            }
+        }
+    }
+}