ci: add frontend and backend workflows

[gac0812]

Summary

• add path-filtered backend CI for locked dependency installation, Ruff, formatting, mypy, pytest, and package builds
• add path-filtered frontend CI for npm installation, code-quality checks, and Android Expo exports
• detect project files so the workflows can land before the backend and frontend PRs

Related PRs

• feat(backend): add FastAPI service #48
• feat(mobile): add Expo React Native app #49
• feat(tooling): add quality check workflow #50

Validation

• inspected the complete workflow diff
• verified the branch contains one Conventional Commit
• verified git diff --check passes

Project-specific steps will activate once the corresponding backend or frontend files are present.

[fennoai]

No issues found in this review. The workflow definitions are internally consistent, intentionally no-op until their project files land, and pass actionlint validation.