Add GET /tags, POST /todos/:id/tags, DELETE /todos/:id/tags/:tag

- GET /tags returns deduplicated sorted array of all tags across todos
- POST /todos/:id/tags attaches a tag to a todo (idempotent, ignores duplicates)
- DELETE /todos/:id/tags/:tag removes a specific tag from a todo
- Both /todos/:id/tags routes return 404 for non-existent todos

index.js

@@ -47,6 +47,34 @@ app.delete('/todos/:id', (req, res) => {
   res.status(204).end();
 });
 
+app.get('/tags', (req, res) => {
+  const tags = [...new Set(todos.flatMap(t => t.tags))].sort();
+  res.json(tags);
+});
+
+app.post('/todos/:id/tags', (req, res) => {
+  const id = parseInt(req.params.id, 10);
+  const todo = todos.find(t => t.id === id);
+  if (!todo) {
+    return res.status(404).end();
+  }
+  const { tag } = req.body;
+  if (!todo.tags.includes(tag)) {
+    todo.tags.push(tag);
+  }
+  res.json(todo);
+});
+
+app.delete('/todos/:id/tags/:tag', (req, res) => {
+  const id = parseInt(req.params.id, 10);
+  const todo = todos.find(t => t.id === id);
+  if (!todo) {
+    return res.status(404).end();
+  }
+  todo.tags = todo.tags.filter(t => t !== req.params.tag);
+  res.status(204).end();
+});
+
 app.listen(PORT, () => {
   console.log(`Server listening on http://localhost:${PORT}`);
 });