ci(monorepo): create a GitHub release for each tagged version

[gustmrg]

Summary

• The Release workflow now creates a GitHub release right after pushing the tag, titled BitFinance Backend vX.Y.Z, BitFinance Frontend vX.Y.Z or BitFinance MCP Server vX.Y.Z.
• Release notes are generated from the commits since the previous tag of the same app.
• Documents the step in docs/deployment.md.

Notes

• Only applies to future releases; existing tags are not backfilled.
• The previous-tag lookup runs for the first time on the next release.