Alpha: CI — GitHub Actions (build + lint + test) for backend and frontend

[coderabbitai]

Context / backlinks:

• Requested by @shiv7shukla on March 17, 2026
• PR added signupcontroller #1: added signupcontroller #1

Goal:
Automate basic quality checks.

Scope:

• Node matrix on LTS
• Install, build TypeScript projects, and run lints
• Placeholder test step to be replaced with real tests later

Acceptance criteria:

• CI runs on PRs and main branch pushes
• Backend and frontend both build successfully
• ESLint runs and fails build on violations