fix: correct version consistency for --find-links to work properly

- All packages now use version 0.2.7 consistently
- Backend packages can find exact leann-core==0.2.7 from local build
- This ensures --find-links works during CI builds instead of falling back to PyPI

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

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

packages/leann-core/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "leann-core"
-version = "0.2.8"
+version = "0.2.7"
 description = "Core API and plugin system for LEANN"
 readme = "README.md"
 requires-python = ">=3.9"