chore(project): license the project under AGPL-3.0

Add the GNU AGPL-3.0 license text, point the openapi.yaml license metadata
at AGPL-3.0-or-later (was the "Proprietary" placeholder), and note the
license in the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -98,3 +98,8 @@ npm run dev                  # Vite dev server
 npm run build                # production build
 npm run generate:types       # regenerate API types from openapi.yaml
 ```
+
+## License
+
+Licensed under the GNU Affero General Public License v3.0 or later
+(AGPL-3.0-or-later). See [`LICENSE`](LICENSE) for the full text.