Add tags field to todos and ?tag= filter on GET /todos

- Add tags:[] to all seed todo objects
- POST /todos accepts optional tags array (defaults to [])
- GET /todos?tag=<value> filters to todos whose tags include that value (case-sensitive)

Closes #14

package.json

@@ -0,0 +1,12 @@
+{
+  "name": "todo-app",
+  "version": "0.1.0",
+  "description": "Minimal Express TODO API",
+  "main": "index.js",
+  "scripts": {
+    "start": "node index.js"
+  },
+  "dependencies": {
+    "express": "^4.18.2"
+  }
+}