chore(release): prepare v0.1.2 release metadata and notes

[weijt606]

Summary

This PR prepares the v0.1.2 release by adding release notes and aligning version metadata across the project.

Changes

• Added v0.1.2 release notes in v0.1.2.md.
• Added the 0.1.2 changelog entry in CHANGELOG.md.
• Bumped project version from 0.1.1 to 0.1.2 in pyproject.toml.
• Updated API version metadata to 0.1.2 in main.py.
• Updated the README release notes link to point to v0.1.2 in README.md.

Why

• Keep release artifacts consistent before merging dev into main.
• Ensure runtime version reporting matches package release version.
• Provide a complete release notes source for GitHub Release creation.

Compatibility

• Breaking changes: None.
• Database migrations: None.
• API contract changes: None.

Validation

• Verified changed files have no editor diagnostics.
• Full backend and frontend test suites were not rerun in this PR.

Checklist

• Release notes added
• Changelog updated
• Project version bumped
• API version metadata aligned
• README release link updated