feat(jellyfin): add ETag-based conditional HTTP caching for home refresh

Introduce an OkHttp interceptor that caches ETags from home-refresh
endpoints and throws NotModifiedException on 304, allowing callers to
reuse cached data. Wire it into the Jellyfin SDK via a dedicated
@JellyfinSdkClient OkHttpClient. Update API methods to return null on
304, and update InMemoryAppContentRepository to keep existing state
when null is returned. Persist dateLastMediaAdded from /UserViews in
HomeCache to short-circuit unchanged /Items/Latest calls.

core/src/main/java/hu/bbara/purefin/core/jellyfin/JellyfinSdkClient.kt

@@ -0,0 +1,13 @@
+package hu.bbara.purefin.core.jellyfin
+
+import javax.inject.Qualifier
+
+/**
+ * Marks an `OkHttpClient` that is wired into the Jellyfin SDK's `OkHttpFactory`.
+ * Distinct from the unqualified client used for image loading and media streaming
+ * because the SDK has its own auth path and the ETag interceptor should not leak
+ * into non-SDK HTTP calls.
+ */
+@Qualifier
+@Retention(AnnotationRetention.BINARY)
+annotation class JellyfinSdkClient