fix: dynamic VALID_CATEGORIES from DB categories table

Same issue as canteen-asset-tracker — hardcoded category set was
{Furniture, Appliances, Utensils, Equipment, Other}, missing all
vending-machine categories (Bev, Snack, GF Bev, GF Food, etc.).

- admin_server.py: _load_categories() from DB at startup
- cantaloupe_sync.py: reads from categories table instead of
  hardcoded set (was silently downgrading GF Bev→Other on sync!)

cantaloupe_sync.py

@@ -721,10 +721,14 @@ def approve_batch(batch_id: int, request: Request):
                 category = str(item.get("category", "Other")).strip()
                 status = str(item.get("status", "active")).strip()
 
-                # Validate enums
-                VALID_CATEGORIES = {"Furniture", "Appliances", "Utensils & Serveware", "Equipment", "Other"}
+                # Validate enums against DB categories table
                 VALID_STATUSES = {"active", "maintenance", "retired"}
-                if category not in VALID_CATEGORIES:
+                try:
+                    valid_cats = {r[0] for r in conn.execute(
+                        "SELECT name FROM categories").fetchall()}
+                except Exception:
+                    valid_cats = {"Other"}
+                if category not in valid_cats:
                     category = "Other"
                 if status not in VALID_STATUSES:
                     status = "active"