feat(app): enqueue gallery batch concurrently via Future.wait

Parallelize the enqueueForUpload collect loop (preparer's internal
bounded semaphore from the previous task keeps IO in check). Also two
test hardenings from that task's code review: pin that failed and
thrown prepare operations release their concurrency slot (cap=1,
subsequent prepare still completes), and pin the maxConcurrentPrepares
<1 -> 1 clamp (peak 1, no deadlock).

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

apps/mobile/lib/features/uploads/enqueue.dart

@@ -149,16 +149,17 @@ Future<EnqueueResult> enqueueForUpload(
       .where((path) => !skipSet.contains(path))
       .toList();
 
-  var enqueued = 0;
-  for (final path in effective) {
-    final task = await preparer.prepare(
-      accountId: accountId,
-      albumId: albumId,
-      categoryId: categoryId,
-      sourcePath: path,
-    );
-    if (task.state == UploadTaskState.queued) enqueued++;
-  }
+  final results = await Future.wait(
+    effective.map((path) => preparer.prepare(
+          accountId: accountId,
+          albumId: albumId,
+          categoryId: categoryId,
+          sourcePath: path,
+        )),
+  );
+  final enqueued = results
+      .where((task) => task.state == UploadTaskState.queued)
+      .length;
 
   return EnqueueResult(
     enqueued: enqueued,