feat(android): in-app updater (backend/Gitea APK), auto-bump versionCode

The app checks {backend}/api/apk/info on launch and, if a newer versionCode
than BuildConfig.VERSION_CODE is offered, shows an update dialog that
downloads {backend}/api/apk into a single wiped-each-time cache file and
hands it to the system installer (FileProvider + REQUEST_INSTALL_PACKAGES).
No APK hoarding; in-place install preserves data (stable signing key needed).

- backend /api/apk/info now returns versionCode (parsed from the release body).
- publish-apk.mjs auto-increments versionCode, sets versionName, and writes
  versionCode/versionName into the Gitea release body.

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

android/app/src/main/AndroidManifest.xml

@@ -8,6 +8,8 @@
 <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>
+    <!-- In-app updater: install a downloaded APK (user still confirms the install) -->
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES"/>
 
     <application
         android:name=".NovaSonicApp"
@@ -44,6 +46,17 @@
         </service>
 
 
+        <!-- FileProvider — hands the downloaded update APK to the system installer -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths"/>
+        </provider>
+
         <!-- Android Auto -->
         <meta-data
             android:name="com.google.android.gms.car.application"

android/app/src/main/kotlin/com/novasonic/app/data/AppUpdater.kt

@@ -0,0 +1,96 @@
+package com.novasonic.app.data
+
+import android.content.Context
+import android.content.Intent
+import androidx.core.content.FileProvider
+import com.novasonic.app.BuildConfig
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.withContext
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import java.io.File
+import java.net.HttpURLConnection
+import java.net.URL
+
+/**
+ * In-app updater for the sideloaded APK. Talks ONLY to the backend the app already knows:
+ *   GET {backend}/api/apk/info  → is there a newer versionCode than this build?
+ *   GET {backend}/api/apk       → download it (the backend proxies the fixed Gitea release)
+ * The user still confirms the system install (Android requires that for sideloaded APKs).
+ *
+ * No APK hoarding: the download goes to a single file under cacheDir/updates that is wiped before
+ * each download. An update (same package + signature) is an in-place install, so app data / offline
+ * downloads survive — provided the signing key stays stable across builds.
+ */
+object AppUpdater {
+
+    @Serializable
+    data class ApkInfo(
+        val available: Boolean = false,
+        val version: String? = null,
+        val versionCode: Int? = null,
+        val size: Long? = null,
+    )
+
+    private val json = Json { ignoreUnknownKeys = true }
+
+    private val _available = MutableStateFlow<ApkInfo?>(null)
+    val available: StateFlow<ApkInfo?> = _available
+
+    private val _downloading = MutableStateFlow(false)
+    val downloading: StateFlow<Boolean> = _downloading
+
+    // Versions the user waved away this session — don't nag again until app restart.
+    @Volatile private var dismissedCode: Int = -1
+
+    suspend fun check(backendUrl: String) {
+        if (backendUrl.isBlank()) return
+        val info = withContext(Dispatchers.IO) {
+            runCatching {
+                val c = (URL("${backendUrl.trimEnd('/')}/api/apk/info").openConnection() as HttpURLConnection)
+                    .apply { connectTimeout = 8000; readTimeout = 8000 }
+                c.inputStream.bufferedReader().use { it.readText() }
+                    .let { json.decodeFromString<ApkInfo>(it) }
+            }.getOrNull()
+        } ?: return
+        val vc = info.versionCode ?: return   // no reliable integer → don't prompt
+        if (info.available && vc > BuildConfig.VERSION_CODE && vc != dismissedCode) {
+            _available.value = info
+        }
+    }
+
+    fun dismiss() {
+        dismissedCode = _available.value?.versionCode ?: dismissedCode
+        _available.value = null
+    }
+
+    suspend fun downloadAndInstall(context: Context, backendUrl: String, onError: (String) -> Unit) {
+        if (_downloading.value || backendUrl.isBlank()) return
+        _downloading.value = true
+        try {
+            val apk = withContext(Dispatchers.IO) {
+                val dir = File(context.cacheDir, "updates").apply { mkdirs(); listFiles()?.forEach { it.delete() } }
+                val out = File(dir, "NovaSonic-update.apk")
+                val c = (URL("${backendUrl.trimEnd('/')}/api/apk").openConnection() as HttpURLConnection)
+                    .apply { connectTimeout = 15000; readTimeout = 30000; instanceFollowRedirects = true }
+                if (c.responseCode !in 200..299) throw RuntimeException("HTTP ${c.responseCode}")
+                c.inputStream.use { input -> out.outputStream().use { input.copyTo(it) } }
+                out
+            }
+            val uri = FileProvider.getUriForFile(context, "${context.packageName}.fileprovider", apk)
+            context.startActivity(
+                Intent(Intent.ACTION_VIEW).apply {
+                    setDataAndType(uri, "application/vnd.android.package-archive")
+                    addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION or Intent.FLAG_ACTIVITY_NEW_TASK)
+                }
+            )
+            _available.value = null   // installer takes over
+        } catch (e: Exception) {
+            onError(e.message ?: "Download fehlgeschlagen")
+        } finally {
+            _downloading.value = false
+        }
+    }
+}

android/app/src/main/kotlin/com/novasonic/app/ui/AppNavigation.kt

@@ -80,6 +80,29 @@ fun AppNavigation() {
     val appSettings   by repo.appSettingsFlow().collectAsStateWithLifecycle(PrefsStore.AppSettings())
     val lastPlayedSong by repo.lastPlayedSongFlow().collectAsStateWithLifecycle(null)
 
+    // ── In-app updater — offer the newer APK the backend advertises ─────────────
+    val updateInfo by com.novasonic.app.data.AppUpdater.available.collectAsStateWithLifecycle()
+    val updating   by com.novasonic.app.data.AppUpdater.downloading.collectAsStateWithLifecycle()
+    val updateScope = rememberCoroutineScope()
+    LaunchedEffect(appSettings.backendUrl) { com.novasonic.app.data.AppUpdater.check(appSettings.backendUrl) }
+    updateInfo?.let { info ->
+        AlertDialog(
+            onDismissRequest = { com.novasonic.app.data.AppUpdater.dismiss() },
+            title = { Text("Update verfügbar") },
+            text  = { Text("NovaSonic ${info.version ?: ""} ist verfügbar. Jetzt laden und installieren? Deine Downloads und Einstellungen bleiben erhalten.") },
+            confirmButton = {
+                TextButton(enabled = !updating, onClick = {
+                    updateScope.launch {
+                        com.novasonic.app.data.AppUpdater.downloadAndInstall(context, appSettings.backendUrl) { msg ->
+                            android.widget.Toast.makeText(context, msg, android.widget.Toast.LENGTH_LONG).show()
+                        }
+                    }
+                }) { Text(if (updating) "Lädt…" else "Laden & installieren") }
+            },
+            dismissButton = { TextButton(onClick = { com.novasonic.app.data.AppUpdater.dismiss() }) { Text("Später") } },
+        )
+    }
+
     // 30s preview (app-level, so its control bar survives navigation). Starting real playback
     // cancels any active preview so the two don't overlap.
     val previewState by app.previewController.state.collectAsStateWithLifecycle()

android/app/src/main/res/xml/file_paths.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="utf-8"?>
+<paths>
+    <!-- The downloaded update APK lives in cacheDir/updates and is shared with the system installer. -->
+    <cache-path name="updates" path="updates/"/>
+</paths>