feat(android): EventsStream reconnect-with-backoff (audit v3 §4.13)

Previously a mid-session SSE drop (server restart / network blip)
left the stream closed until the next sign-in — cross-device
reactivity (likes, playlist updates, request status, quarantine
changes) silently died until app relaunch.

Now onClosed / onFailure schedule a reconnect with exponential
backoff (1s → 2s → … → 30s cap), reset to 1s on a successful
onOpen. Reconnect only fires while signed in; sign-out's
disconnect() cancels any pending retry. State swaps
(connect/disconnect/scheduleReconnect) are @Synchronized since the
OkHttp listener thread and the auth-cookie collector both touch
currentSource + backoff.

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

android/app/src/main/java/com/fabledsword/minstrel/events/EventsStream.kt

@@ -3,7 +3,9 @@ package com.fabledsword.minstrel.events
 import com.fabledsword.minstrel.auth.AuthStore
 import com.fabledsword.minstrel.di.ApplicationScope
 import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.channels.BufferOverflow
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.flow.asSharedFlow
@@ -26,6 +28,9 @@ import javax.inject.Singleton
 
 private const val SSE_PATH = "/api/events/stream"
 private const val EVENTS_BUFFER_CAPACITY = 64
+private const val BASE_BACKOFF_MS = 1_000L
+private const val MAX_BACKOFF_MS = 30_000L
+private const val BACKOFF_FACTOR = 2
 
 /**
  * Long-lived SSE subscription to `GET /api/events/stream`. Exposes
@@ -40,9 +45,12 @@ private const val EVENTS_BUFFER_CAPACITY = 64
  *   back to null (sign-out).
  * - No client-side timeout — the server emits 15s heartbeats which
  *   okhttp-sse handles transparently.
- * - No explicit reconnect-with-backoff in v1; if the stream drops
- *   mid-session (server restart, network blip) it stays closed until
- *   the next auth-cookie transition. Matches Flutter's behavior.
+ * - Reconnect-with-backoff: if the stream drops mid-session (server
+ *   restart, network blip) it reconnects with exponential backoff
+ *   (1s → 2s → … → 30s cap), reset to 1s on a successful open. Only
+ *   reconnects while still signed in; a sign-out cancels the pending
+ *   retry. Without this a single blip silently kills cross-device
+ *   reactivity until the next app launch.
  *
  * The URL passes through the placeholder host that
  * `BaseUrlInterceptor` rewrites — same mechanism the rest of the
@@ -66,19 +74,59 @@ class EventsStream @Inject constructor(
     val events: SharedFlow<LiveEvent> = emitter.asSharedFlow()
 
     private var currentSource: EventSource? = null
+    @Volatile private var signedIn = false
+    private var reconnectJob: Job? = null
+    private var backoffMs = BASE_BACKOFF_MS
 
     init {
         scope.launch {
             authStore.sessionCookie
                 .map { !it.isNullOrEmpty() }
                 .distinctUntilChanged()
-                .collect { signedIn ->
-                    currentSource?.cancel()
-                    currentSource = if (signedIn) openSubscription() else null
+                .collect { isSignedIn ->
+                    signedIn = isSignedIn
+                    if (isSignedIn) {
+                        backoffMs = BASE_BACKOFF_MS
+                        connect()
+                    } else {
+                        disconnect()
+                    }
                 }
         }
     }
 
+    @Synchronized
+    private fun connect() {
+        reconnectJob?.cancel()
+        currentSource?.cancel()
+        currentSource = openSubscription()
+    }
+
+    @Synchronized
+    private fun disconnect() {
+        reconnectJob?.cancel()
+        currentSource?.cancel()
+        currentSource = null
+    }
+
+    /**
+     * Schedule a reconnect after the current backoff, then double it
+     * (capped). No-op when signed out — sign-out's [disconnect]
+     * cancels the pending job. A successful [Listener.onOpen] resets
+     * the backoff to the floor.
+     */
+    @Synchronized
+    private fun scheduleReconnect() {
+        if (!signedIn) return
+        reconnectJob?.cancel()
+        val waitMs = backoffMs
+        backoffMs = (backoffMs * BACKOFF_FACTOR).coerceAtMost(MAX_BACKOFF_MS)
+        reconnectJob = scope.launch {
+            delay(waitMs)
+            if (signedIn) connect()
+        }
+    }
+
     private fun openSubscription(): EventSource {
         val request = Request.Builder()
             .url("http://placeholder.invalid$SSE_PATH")
@@ -88,6 +136,10 @@ class EventsStream @Inject constructor(
     }
 
     private inner class Listener : EventSourceListener() {
+        override fun onOpen(eventSource: EventSource, response: Response) {
+            backoffMs = BASE_BACKOFF_MS
+        }
+
         override fun onEvent(
             eventSource: EventSource,
             id: String?,
@@ -98,8 +150,7 @@ class EventsStream @Inject constructor(
         }
 
         override fun onClosed(eventSource: EventSource) {
-            // Stream ended (server-side close or network drop). No
-            // explicit reconnect — auth-cookie transitions re-open.
+            scheduleReconnect()
         }
 
         override fun onFailure(
@@ -107,9 +158,7 @@ class EventsStream @Inject constructor(
             t: Throwable?,
             response: Response?,
         ) {
-            // Same as onClosed — auth transition will re-open. The
-            // throwable/response carry diagnostics; we don't surface
-            // them in v1 since the stream is "best effort" by design.
+            scheduleReconnect()
         }
     }