refactor(skills): remove subcategory field from skills

Drop subcategory column from database schema and update constraints
Remove subcategory handling from backend queries and API endpoints
Clean up frontend components and admin views to reflect the change

frontend/src/views/HabitatEdit.vue

@@ -138,7 +138,7 @@ async function createMultiOption(selectKey: string, type: ConfigType, name: stri
   creatingSelect.value = selectKey;
   message.value = '';
   try {
-    const created = await api.createConfig(type, { name: cleanName, subcategory: null });
+    const created = await api.createConfig(type, { name: cleanName });
     await loadOptions();
     const value = String(created.id);
     if (!values.includes(value)) {