updated CHANGELOG.md and added missing type hints

[StevenBtw]

Summary by cubic

Adds missing type hints and richer docstrings to the graflo.db.grafeo backend, plus a small internal helper for building edge MERGE queries. Updates CHANGELOG with v1.8.0 notes highlighting the embedded GrafeoConnection/GrafeoConfig backend and grafeo>=0.5.33.

• Refactors
  • Added return type hints (e.g., -> None) across Grafeo connection methods.
  • Expanded docstrings with clear Args/Returns and clarified no-ops.
  • Extracted _build_edge_merge_query and used it in insert_edges_batch for readability; behavior unchanged.

^{Written for commit 1c52068. Summary will update on new commits.}