release: Bump version to 1.3.1

Promote v1.3.1-rc.1 to stable release "Syndicate Tags"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

starpunk/__init__.py

@@ -325,5 +325,5 @@ def create_app(config=None):
 
 # Package version (Semantic Versioning 2.0.0)
 # See docs/standards/versioning-strategy.md for details
-__version__ = "1.3.1rc1"
+__version__ = "1.3.1"
-__version_info__ = (1, 3, 1, "rc1")
+__version_info__ = (1, 3, 1)