Phase 0: Fix test infrastructure

- Fix critical bug in createTestUserOnce() (dead code, wrong return type)
- Add test_helpers_db.go with 6 new helper functions
- Impact: All tests can now create users reliably

cmd/server/tests/sync_integration_test.go

@@ -183,3 +183,50 @@ func TestSyncIntegration_QueueProcessor_EnqueueProgress(t *testing.T) {
 	assert.Equal(t, sync.SyncStatusPending, item.Status.String)
 	assert.Equal(t, int32(sync.PriorityPageTurn), item.Priority.Int32)
 }
+
+// PHASE 2: Concurrency Protection
+// TestSyncConcurrent_ProgressUpdates tests multiple devices updating same book simultaneously
+func TestSyncConcurrent_ProgressUpdates(t *testing.T) {
+	ctx := context.Background()
+	db := setupSyncTestDB(t)
+	userID := createSyncTestUser(t, db)
+	deviceID := createSyncTestDevice(t, db, userID)
+	mediaItemID := createSyncTestMedia(t, db)
+
+	// Progress values that will be updated concurrently
+	progressValues := []float64{25.0, 50.0, 75.0}
+
+	// Define update operations
+	var updateOps []func() error
+	for _, progress := range progressValues {
+		p := progress
+		updateOps = append(updateOps, func() error {
+			update := &sync.ProgressUpdate{
+				DeviceID:    pgtype.UUID{Bytes: [16]byte(deviceID), Valid: true},
+				MediaItemID: pgtype.UUID{Bytes: [16]byte(mediaItemID), Valid: true},
+				UserID:      pgtype.UUID{Bytes: [16]byte(userID), Valid: true},
+				Percentage:  p,
+				Source:      "koreader",
+			}
+			processor := sync.NewSyncQueueProcessor(db)
+			return processor.EnqueueProgress(update)
+		})
+	}
+
+	// Execute updates concurrently
+	errors := runConcurrent(t, len(updateOps), updateOps)
+	for err := range errors {
+		t.Logf("Concurrent update error: %v", err)
+	}
+
+	// PHASE 2: Database verification
+	// Verify final database state is consistent
+	// With concurrent updates, one should win - verify database has one value
+	progress, err := db.GetReadingProgress(ctx, database.GetReadingProgressParams{
+		MediaItemID: pgtype.UUID{Bytes: [16]byte(mediaItemID), Valid: true},
+		UserID:      pgtype.UUID{Bytes: [16]byte(userID), Valid: true},
+	})
+	require.NoError(t, err, "should retrieve final reading progress")
+	assert.True(t, progress.Percentage.Valid, "percentage should be set")
+	assert.Contains(t, progressValues, progress.Percentage.Float64, "final percentage should match one of the concurrent updates")
+}