fix: correct UUID format string issues in logging and tests

- Fix scheduler.go log.Printf calls to convert pgtype.UUID to string before formatting
- Fix ebook.go fmt.Printf calls to convert pgtype.UUID to string before formatting
- Add missing Enabled field to rate limiter config in security test
- Prevents format string errors when logging library IDs

This resolves compilation errors where pgtype.UUID was being formatted
with %s which expects a string, not a UUID struct.

internal/handlers/ebook.go

@@ -797,11 +797,12 @@ func (h *Handler) StartWatchModeForAllLibraries(ctx context.Context) error {
 	}
 
 	for _, library := range libraries {
+		libraryIDStr := fmt.Sprintf("%x", library.ID.Bytes)
 		if err := h.StartWatchModeForLibrary(ctx, library.ID, library.ID); err != nil {
-			fmt.Printf("Warning: failed to start watch mode for library %s: %v\n", library.ID, err)
+			fmt.Printf("Warning: failed to start watch mode for library %s: %v\n", libraryIDStr, err)
 			continue
 		}
-		fmt.Printf("Started watch mode for library %s (%s)\n", library.Name, library.ID)
+		fmt.Printf("Started watch mode for library %s (%s)\n", library.Name, libraryIDStr)
 	}
 
 	return nil