chore(license): relicense project from GPL-3.0 to AGPL-3.0

Replace the GPL-3.0 license text with the full GNU Affero General
Public License v3.0 text, strengthening copyleft coverage for the
network-service use case (users interacting with Bookhoard over the
network are entitled to the corresponding source).

- LICENSE: swap GPL-3.0 text for the canonical AGPL-3.0 text (gnu.org)
- README.md: update both license references (Project Status and
  License sections) from GPL-3.0 to AGPL-3.0
- docs/user/sync-guide.md: update the footer license reference

The bundled BSD 3-Clause license in internal/sevenzip/LICENSE is a
third-party dependency license and is intentionally left unchanged.

README.md

@@ -155,7 +155,7 @@ bruno run
 ## 📊 Project Status
 
 **Version**: 1.0  
-**License**: GPL-3.0  
+**License**: AGPL-3.0  
 **Status**: Production-ready ✅
 
 ---
@@ -168,7 +168,7 @@ We welcome contributions! Please see [docs/developer/development.md](docs/develo
 
 ## 📄 License
 
-GPL-3.0 - See [LICENSE](LICENSE) file for details.
+AGPL-3.0 - See [LICENSE](LICENSE) file for details.
 
 ---