fix(mobile): bound queued media authentication work

Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/shared/relay/media_auth.dart

@@ -37,6 +37,9 @@ class MediaGetAuthService {
 
   Map<String, String>? _cachedHeaders;
   DateTime? _refreshAt;
+  // Includes the running job. Bound retained work during rapid history scrolls.
+  static const _maxPendingSignatures = 64;
+  int _pendingSignatures = 0;
   Future<void> _signingTail = Future<void>.value();
 
   MediaGetAuthService({
@@ -58,12 +61,18 @@ class MediaGetAuthService {
   ///
   /// Image loading starts during layout. Serialize signing per auth scope so a
   /// viewport full of uncached images does not spawn an isolate per row at once.
-  /// Timestamp each proof when its queued job starts, not when it was enqueued.
+  /// At most 64 requests may be outstanding. Excess requests fail explicitly
+  /// without spawning or retaining work; image failures are evicted so a later
+  /// resolution can retry. Timestamp proofs when their queued jobs start.
   Future<Map<String, String>> headersForAsync(String url) {
     final nsec = _nsec;
     if (nsec == null || nsec.isEmpty || !isRelayMediaUrl(url)) {
       return Future.value(const {});
     }
+    if (_pendingSignatures >= _maxPendingSignatures) {
+      return Future.error(StateError('Media authentication queue is full'));
+    }
+    _pendingSignatures++;
     final released = Completer<void>();
     final result = _signingTail.then((_) async {
       try {
@@ -76,6 +85,7 @@ class MediaGetAuthService {
       } finally {
         // A failed worker must propagate to its fetch without blocking later
         // requests in the queue.
+        _pendingSignatures--;
         released.complete();
       }
     });

mobile/test/shared/relay/media_image_test.dart

@@ -102,6 +102,30 @@ void main() {
       },
     );
 
+    test('bounds queued signing and admits requests after it drains', () async {
+      final auth = _auth(nsec: nostr.Keys.generate().nsec);
+      // Enqueue in one turn, before a worker can finish. Record every failure
+      // immediately so overflow futures cannot become unhandled errors.
+      final requests = List.generate(
+        80,
+        (_) => auth
+            .headersForAsync(_mediaUrl)
+            .then<Object>(
+              (headers) => headers,
+              onError: (Object error) => error,
+            ),
+      );
+      final results = await Future.wait(requests);
+      expect(results.whereType<Map<String, String>>(), hasLength(64));
+      final rejected = results.whereType<StateError>().toList();
+      expect(rejected, hasLength(16));
+      expect(
+        rejected.map((error) => error.message),
+        everyElement('Media authentication queue is full'),
+      );
+      expect(await auth.headersForAsync(_mediaUrl), isNotEmpty);
+    });
+
     test('keeps the signing identity separate across auth scopes', () async {
       final first = nostr.Keys.generate();
       final second = nostr.Keys.generate();