fix(android): surface concert-fetch errors instead of swallowing them

Events were fetched with runCatching{}.getOrDefault(emptyList()), so any
network/deserialization failure looked identical to "no concerts". The
backend + Ticketmaster work (verified), so the failure is client-side.

- EventsClient: expectSuccess + timeouts so a non-2xx (e.g. an SSO/HTML
  redirect) becomes a real exception instead of an obscure body() failure.
- ArtistViewModel: capture the failure into eventsError (+ Logcat) rather
  than defaulting to empty.
- ConcertsSheet: show "Konzerte konnten nicht geladen werden" + the reason
  when the fetch failed, vs. "Keine Konzerte angekündigt" when genuinely none.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/api/EventsClient.kt

@@ -4,6 +4,7 @@ import com.novasonic.app.api.model.ConcertEvent
 import io.ktor.client.*
 import io.ktor.client.call.*
 import io.ktor.client.engine.android.*
+import io.ktor.client.plugins.*
 import io.ktor.client.plugins.contentnegotiation.*
 import io.ktor.client.request.*
 import io.ktor.serialization.kotlinx.json.*
@@ -12,6 +13,10 @@ import kotlinx.serialization.json.Json
 object EventsClient {
 
     private val http = HttpClient(Android) {
+        // Surface non-2xx as a real exception (e.g. an SSO redirect / HTML login page) instead of
+        // letting .body() fail obscurely — the ViewModel turns it into a visible "load failed".
+        expectSuccess = true
+        install(HttpTimeout) { requestTimeoutMillis = 15_000; connectTimeoutMillis = 10_000 }
         install(ContentNegotiation) {
             json(Json { ignoreUnknownKeys = true; coerceInputValues = true })
         }

android/app/src/main/kotlin/com/novasonic/app/ui/component/ConcertsSheet.kt

@@ -31,6 +31,7 @@ fun ConcertsSheet(
     artistName: String,
     events: List<ConcertEvent>,
     onDismiss: () -> Unit,
+    eventsError: String? = null,
 ) {
     val context = LocalContext.current
 
@@ -47,17 +48,25 @@ fun ConcertsSheet(
         )
 
         if (events.isEmpty()) {
-            Box(
-                contentAlignment = Alignment.Center,
+            Column(
+                horizontalAlignment = Alignment.CenterHorizontally,
                 modifier = Modifier
                     .fillMaxWidth()
-                    .padding(vertical = 40.dp),
+                    .padding(horizontal = 20.dp, vertical = 40.dp),
             ) {
                 Text(
-                    "Keine Konzerte angekündigt",
+                    if (eventsError != null) "Konzerte konnten nicht geladen werden" else "Keine Konzerte angekündigt",
                     fontFamily = Outfit, fontSize = 14.sp,
                     color = NsOnSurface2,
                 )
+                if (eventsError != null) {
+                    Spacer(Modifier.height(6.dp))
+                    Text(
+                        eventsError,
+                        fontFamily = Outfit, fontSize = 11.sp,
+                        color = NsOnSurface2.copy(alpha = 0.6f),
+                    )
+                }
             }
         } else {
             LazyColumn(

android/app/src/main/kotlin/com/novasonic/app/ui/screen/ArtistDetailScreen.kt

@@ -92,9 +92,10 @@ fun ArtistDetailScreen(
         val artist = state.artist
         if (artist != null) {
             ConcertsSheet(
-                artistName = artist.name,
-                events     = state.events,
-                onDismiss  = vm::dismissEventsSheet,
+                artistName  = artist.name,
+                events      = state.events,
+                onDismiss   = vm::dismissEventsSheet,
+                eventsError = state.eventsError,
             )
         }
     }

android/app/src/main/kotlin/com/novasonic/app/viewmodel/ArtistViewModel.kt

@@ -41,6 +41,7 @@ data class ArtistUiState(
     val partialAlbum: PartialAlbumState? = null,
     // Concerts
     val events: List<ConcertEvent> = emptyList(),
+    val eventsError: String? = null,     // non-null → the fetch failed (vs. genuinely no concerts)
     val showEventsSheet: Boolean = false,
     // Deezer fan count of the matched artist (shown under the name), null if unknown
     val fanCount: Long? = null,
@@ -113,8 +114,12 @@ class ArtistViewModel(app: Application, savedState: SavedStateHandle) : AndroidV
                 launch { loadItunesData(artist.name, pin) }
                 launch {
                     val backendUrl = repo.getBackendUrl()
-                    val ev = runCatching { EventsClient.fetchEvents(backendUrl, artist.name) }.getOrDefault(emptyList())
-                    _state.value = _state.value.copy(events = ev)
+                    runCatching { EventsClient.fetchEvents(backendUrl, artist.name) }
+                        .onSuccess  { _state.value = _state.value.copy(events = it, eventsError = null) }
+                        .onFailure  { e ->
+                            android.util.Log.w("NovaSonic", "Konzert-Fetch fehlgeschlagen für ${artist.name}", e)
+                            _state.value = _state.value.copy(events = emptyList(), eventsError = e.message ?: e.toString())
+                        }
                 }
             } catch (e: Exception) {
                 _state.value = ArtistUiState(isLoading = false, itunesLoading = false, error = e.message ?: e.toString())