fix(sync): prevent nil pointer dereference when existing progress is missing

In applyProgressResolution and applyResolution, currentPage and totalPages
were unconditionally read from existingProgress even when the preceding
query returned err (no rows). This caused a nil pointer dereference when
no existing reading progress existed for a media item. Now declare the
variables as zero-value pgtype.Int4 and only populate them from
existingProgress when err is nil.

internal/handlers/conflicts.go

@@ -316,8 +316,12 @@ func (h *ConflictHandler) applyProgressResolution(mediaItemID pgtype.UUID, userI
 		characterOffset = pgtype.Int8{Int64: int64(c), Valid: true}
 	}
 
-	currentPage := existingProgress.CurrentPage
-	totalPages := existingProgress.TotalPages
+	var currentPage pgtype.Int4
+	var totalPages pgtype.Int4
+	if err == nil {
+		currentPage = existingProgress.CurrentPage
+		totalPages = existingProgress.TotalPages
+	}
 
 	if p, ok := data["page"].(float64); ok {
 		currentPage = pgtype.Int4{Int32: int32(p), Valid: true}
@@ -661,8 +665,12 @@ func (h *ConflictHandler) applyResolution(mediaItemID pgtype.UUID, userID pgtype
 		characterOffset = pgtype.Int8{Int64: int64(c), Valid: true}
 	}
 
-	currentPage := existingProgress.CurrentPage
-	totalPages := existingProgress.TotalPages
+	var currentPage pgtype.Int4
+	var totalPages pgtype.Int4
+	if err == nil {
+		currentPage = existingProgress.CurrentPage
+		totalPages = existingProgress.TotalPages
+	}
 
 	if p, ok := data["page"].(float64); ok {
 		currentPage = pgtype.Int4{Int32: int32(p), Valid: true}