fix(categories): validate category name length instead of 500ing

photo_categories.name is varchar(100). Neither the input nor the route
checked length, so a 267-char name hit a raw Postgres "value too long",
came back as a 500, and the form silently stayed open with no toast.

Add isLength({ max: 100 }) to POST / and PUT /:id (the update route had the
identical gap) so it returns the route family's normal 400 { errors: [...] }
shape that the toast helper already renders, and maxLength={100} on the three
category-name inputs (create + inline edit in CategoryManager, create in
EventCategoryManager).

Refs testplan REPORT.md #4 (Part 7.01).

frontend/src/components/admin/CategoryManager.tsx

@@ -138,6 +138,7 @@ export const CategoryManager: React.FC = () => {
             onChange={(e) => setNewCategoryName(e.target.value)}
             onKeyPress={(e) => e.key === 'Enter' && handleCreate()}
             placeholder={t('categories.categoryName')}
+            maxLength={100}
             className="flex-1 px-3 py-2 border border-neutral-300 dark:border-neutral-600 rounded-md bg-white dark:bg-neutral-800 text-neutral-900 dark:text-neutral-100 focus:ring-2 focus:ring-primary-500"
             autoFocus
           />
@@ -188,6 +189,7 @@ export const CategoryManager: React.FC = () => {
                       if (e.key === 'Enter') handleUpdate(category.id);
                       if (e.key === 'Escape') cancelEdit();
                     }}
+                    maxLength={100}
                     className="flex-1 px-3 py-1 border border-neutral-300 dark:border-neutral-600 rounded-md bg-white dark:bg-neutral-800 text-neutral-900 dark:text-neutral-100 focus:ring-2 focus:ring-primary-500"
                     autoFocus
                   />

frontend/src/components/admin/EventCategoryManager.tsx

@@ -206,6 +206,7 @@ export const EventCategoryManager: React.FC<EventCategoryManagerProps> = ({ even
             onChange={(e) => setNewCategoryName(e.target.value)}
             onKeyPress={(e) => e.key === 'Enter' && handleCreate()}
             placeholder={t('categories.categoryName')}
+            maxLength={100}
             className="flex-1 px-3 py-1.5 text-sm border border-neutral-300 dark:border-neutral-600 rounded-md bg-white dark:bg-neutral-800 text-neutral-900 dark:text-neutral-100 focus:ring-2 focus:ring-primary-500"
             autoFocus
           />