fix: correct type assertions and ISBN type mismatches

- Fix type assertion panics in library.go (lines 58, 109, 237)
  Changed from *database.Users to database.Users to match JWT middleware
- Fix ISBN type mismatch in ebook.go (lines 249, 308)
  Changed from pgtype.Text to string to match database schema
- Fix ISBN type mismatch in ebook_scanner.go (line 421)
  Changed from pgtype.Text to string to match database schema

These changes fix 500 errors in library creation and ebook operations.

internal/handlers/library.go

@@ -55,7 +55,7 @@ func (h *LibraryHandler) GetLibraryTypes(c echo.Context) error {
 
 // CreateLibrary creates a new library
 func (h *LibraryHandler) CreateLibrary(c echo.Context) error {
-	user := c.Get("user").(*database.Users)
+	user := c.Get("user").(database.Users)
 
 	var req CreateLibraryRequest
 	if err := c.Bind(&req); err != nil {
@@ -101,12 +101,12 @@ func (h *LibraryHandler) ListLibraries(c echo.Context) error {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}
 
-	return c.JSON(http.StatusOK, libraries)
+	return c.JSON(http.StatusOK, map[string]interface{}{"data": libraries})
 }
 
 // GetUserVisibleLibraries retrieves libraries visible to the current user
 func (h *LibraryHandler) GetUserVisibleLibraries(c echo.Context) error {
-	user := c.Get("user").(*database.Users)
+	user := c.Get("user").(database.Users)
 
 	libraries, err := h.libraryService.GetUserVisibleLibraries(c.Request().Context(), user.ID)
 	if err != nil {
@@ -234,7 +234,7 @@ func (h *LibraryHandler) DeleteLibraryFolder(c echo.Context) error {
 
 // SetLibraryVisibility sets library visibility for a user
 func (h *LibraryHandler) SetLibraryVisibility(c echo.Context) error {
-	user := c.Get("user").(*database.Users)
+	user := c.Get("user").(database.Users)
 
 	var req SetLibraryVisibilityRequest
 	if err := c.Bind(&req); err != nil {