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).

backend/src/routes/adminCategories.js

@@ -40,7 +40,11 @@ router.get('/event/:eventId', adminAuth, requirePermission('settings.view'), req
 
 // Create a new category
 router.post('/', adminAuth, requirePermission('settings.edit'), [
-  body('name').notEmpty().withMessage('Category name is required'),
+  // photo_categories.name is varchar(100) — without the length check Postgres
+  // raises "value too long" and the catch below turns it into a raw 500 with
+  // no usable message for the form.
+  body('name').notEmpty().withMessage('Category name is required')
+    .isLength({ max: 100 }).withMessage('Category name must be at most 100 characters'),
   body('slug').optional(),
   body('is_global').optional().isBoolean(),
   body('event_id').optional().isInt(),
@@ -127,7 +131,8 @@ router.post('/', adminAuth, requirePermission('settings.edit'), [
 
 // Update a category
 router.put('/:id', adminAuth, requirePermission('settings.edit'), [
-  body('name').notEmpty().withMessage('Category name is required'),
+  body('name').notEmpty().withMessage('Category name is required')
+    .isLength({ max: 100 }).withMessage('Category name must be at most 100 characters'),
   body('hero_photo_id').optional({ nullable: true }).custom((value) => {
     if (value === null || value === undefined) return true;
     return Number.isInteger(Number(value));