feat(android): VersionGate banner (audit v2 #23)

* HealthzApi — GET /healthz unauthenticated endpoint returning
  {status, version, min_client_version}.
* VersionCheckController — Hilt singleton; 5-minute poll loop on
  appScope, soft-fails on network errors (keeps last-known
  VersionResult: OK / TOO_OLD / SKIPPED). Reuses isVersionNewer()
  from the About card's UpdateRepository. Exposes recheck() for
  the banner's "Check now" button.
* VersionTooOldBanner — shell-level Compose banner with
  AnimatedVisibility shrink/expand, triangle-alert icon, copy
  matching Flutter, "Check now" trailing button. Tiny
  VersionTooOldViewModel lifts the StateFlow through Hilt.
* ShellScaffold adds VersionTooOldBanner() above ConnectionErrorBanner()
  in the existing banner slot.
* MinstrelApplication uses the construct-the-singleton trick to
  start the poll loop at app launch.

Closes audit v2 #23. Locally cached content keeps working when
the banner is shown — the message nudges the user toward an
update without blocking the rest of the UI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/MinstrelApplication.kt

@@ -14,6 +14,7 @@ import com.fabledsword.minstrel.events.LiveEventsDispatcher
 import com.fabledsword.minstrel.player.CoverPrefetcher
 import com.fabledsword.minstrel.player.PlayEventsReporter
 import com.fabledsword.minstrel.player.ResumeController
+import com.fabledsword.minstrel.update.data.VersionCheckController
 import dagger.hilt.android.HiltAndroidApp
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.launch
@@ -79,6 +80,14 @@ class MinstrelApplication :
      */
     @Suppress("unused") @Inject lateinit var coverPrefetcher: CoverPrefetcher
 
+    /**
+     * Same construct-the-singleton trick — VersionCheckController's
+     * init block starts a 5-min poll loop against /healthz so the
+     * shell-level VersionTooOldBanner can surface min_client_version
+     * mismatches without waiting for the next user-driven request.
+     */
+    @Suppress("unused") @Inject lateinit var versionCheckController: VersionCheckController
+
     /**
      * Same construct-the-singleton trick — EventsStream's init block
      * subscribes to AuthStore.sessionCookie and opens / closes the

android/app/src/main/java/com/fabledsword/minstrel/shared/widgets/ShellScaffold.kt

@@ -16,6 +16,7 @@ import com.fabledsword.minstrel.connectivity.ui.ConnectionErrorBanner
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
 import com.fabledsword.minstrel.player.ui.MiniPlayer
+import com.fabledsword.minstrel.update.ui.VersionTooOldBanner
 import com.fabledsword.minstrel.shared.widgets.trackactions.TrackActionsViewModel
 
 /**
@@ -53,9 +54,11 @@ fun ShellScaffold(
         }
     }
     Column(modifier = modifier.fillMaxSize()) {
-        // Banner slot. ConnectionErrorBanner auto-shows when the device
+        // Banner slot. VersionTooOldBanner appears when the server's
-        // loses usable internet. VersionTooOld / UpdateBanner will join
+        // min_client_version exceeds our build; ConnectionErrorBanner
-        // this slot once those mechanisms exist.
+        // when the device loses usable internet. UpdateBanner will
+        // join this slot once #25 ships.
+        VersionTooOldBanner()
         ConnectionErrorBanner()
         Box(modifier = Modifier.fillMaxWidth().weight(1f)) {
             content()

android/app/src/main/java/com/fabledsword/minstrel/update/api/HealthzApi.kt

@@ -0,0 +1,28 @@
+package com.fabledsword.minstrel.update.api
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import retrofit2.http.GET
+
+/**
+ * Retrofit interface for `GET /healthz` — unauthenticated health probe
+ * returning the server's running version + the minimum client version
+ * it'll talk to. Used by [com.fabledsword.minstrel.update.data.VersionCheckController]
+ * to surface the VersionTooOld banner.
+ */
+interface HealthzApi {
+    @GET("healthz")
+    suspend fun check(): HealthzResponse
+}
+
+/**
+ * Wire shape for /healthz. `minClientVersion` may be empty on older
+ * servers; the caller treats empty as "skip" rather than "fail" so a
+ * partial deploy doesn't gate the UI.
+ */
+@Serializable
+data class HealthzResponse(
+    val status: String = "",
+    val version: String = "",
+    @SerialName("min_client_version") val minClientVersion: String = "",
+)

android/app/src/main/java/com/fabledsword/minstrel/update/data/VersionCheckController.kt

@@ -0,0 +1,67 @@
+package com.fabledsword.minstrel.update.data
+
+import com.fabledsword.minstrel.BuildConfig
+import com.fabledsword.minstrel.di.ApplicationScope
+import com.fabledsword.minstrel.update.api.HealthzApi
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.launch
+import retrofit2.Retrofit
+import javax.inject.Inject
+import javax.inject.Singleton
+
+private const val POLL_INTERVAL_MS = 5 * 60 * 1000L
+
+/**
+ * Result of the most recent /healthz version-compatibility check.
+ * `Skipped` means the server didn't include `min_client_version`
+ * (partial deploy or older server) and the UI should not gate.
+ */
+enum class VersionResult { OK, TOO_OLD, SKIPPED }
+
+/**
+ * Polls /healthz periodically and exposes the current
+ * [VersionResult] for the shell's VersionTooOld banner. Soft-fails
+ * on network errors — keeps the last-known result rather than
+ * showing a misleading "too old" on a connectivity blip.
+ *
+ * Constructed at app launch via the construct-the-singleton trick
+ * in [com.fabledsword.minstrel.MinstrelApplication].
+ */
+@Singleton
+class VersionCheckController @Inject constructor(
+    @ApplicationScope private val scope: CoroutineScope,
+    retrofit: Retrofit,
+) {
+    private val api: HealthzApi = retrofit.create(HealthzApi::class.java)
+
+    private val internal = MutableStateFlow(VersionResult.SKIPPED)
+    val result: StateFlow<VersionResult> = internal.asStateFlow()
+
+    init {
+        scope.launch {
+            while (true) {
+                runOnce()
+                delay(POLL_INTERVAL_MS)
+            }
+        }
+    }
+
+    /** One-shot recheck, bypassing the poll cadence. Used by the banner's "Check now" button. */
+    fun recheck() {
+        scope.launch { runOnce() }
+    }
+
+    private suspend fun runOnce() {
+        val response = runCatching { api.check() }.getOrNull() ?: return
+        val min = response.minClientVersion
+        internal.value = when {
+            min.isEmpty() -> VersionResult.SKIPPED
+            isVersionNewer(min, BuildConfig.VERSION_NAME) -> VersionResult.TOO_OLD
+            else -> VersionResult.OK
+        }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/update/ui/VersionTooOldBanner.kt

@@ -0,0 +1,89 @@
+package com.fabledsword.minstrel.update.ui
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.animation.expandVertically
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.shrinkVertically
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Icon
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import androidx.hilt.navigation.compose.hiltViewModel
+import androidx.lifecycle.SavedStateHandle
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import com.composables.icons.lucide.Lucide
+import com.composables.icons.lucide.TriangleAlert
+import com.fabledsword.minstrel.update.data.VersionCheckController
+import com.fabledsword.minstrel.update.data.VersionResult
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.StateFlow
+import javax.inject.Inject
+
+/**
+ * Tiny VM that exposes the [VersionCheckController]'s current
+ * [VersionResult] plus its recheck trigger for the banner button.
+ */
+@HiltViewModel
+class VersionTooOldViewModel @Inject constructor(
+    private val controller: VersionCheckController,
+    @Suppress("UnusedPrivateProperty") savedStateHandle: SavedStateHandle,
+) : ViewModel() {
+    val result: StateFlow<VersionResult> = controller.result
+    fun recheck() = controller.recheck()
+}
+
+/**
+ * Shell-level banner shown when the server reports `min_client_version`
+ * newer than our build. Non-blocking — locally cached content keeps
+ * working; the banner is the nudge to install the next APK. "Check
+ * now" forces a fresh /healthz probe.
+ */
+@Composable
+fun VersionTooOldBanner(viewModel: VersionTooOldViewModel = hiltViewModel()) {
+    val result by viewModel.result.collectAsStateWithLifecycle()
+    AnimatedVisibility(
+        visible = result == VersionResult.TOO_OLD,
+        enter = expandVertically() + fadeIn(),
+        exit = shrinkVertically() + fadeOut(),
+    ) {
+        Row(
+            modifier = Modifier
+                .fillMaxWidth()
+                .background(MaterialTheme.colorScheme.errorContainer)
+                .padding(horizontal = 16.dp, vertical = 10.dp),
+            verticalAlignment = Alignment.CenterVertically,
+            horizontalArrangement = Arrangement.spacedBy(8.dp),
+        ) {
+            Icon(
+                imageVector = Lucide.TriangleAlert,
+                contentDescription = null,
+                tint = MaterialTheme.colorScheme.onErrorContainer,
+            )
+            Text(
+                text = "This app is older than the server requires. " +
+                    "Cached content still works; install an update when ready.",
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onErrorContainer,
+                modifier = Modifier.weight(1f),
+            )
+            TextButton(onClick = { viewModel.recheck() }) {
+                Text(
+                    text = "Check now",
+                    color = MaterialTheme.colorScheme.onErrorContainer,
+                )
+            }
+        }
+    }
+}