ci: set up GitHub Actions CI workflow (T431224)

[pushpaktiwarii]

Description

Resolves task T431224.

This PR introduces a GitHub Actions CI workflow that automatically installs dependencies, runs backend tests, and builds the frontend on pull requests.

While setting up the CI, I also fixed the backend test environment so the test suite can run successfully without any local dependencies:

• Added test-only JWT secrets in setup.js to prevent auth-related failures.
• Removed a stale import (wikimediaOAuth1Routes) in testApp.js referencing a deleted route.
• Minor formatting fix in .env.example.

Acceptance Criteria Met

• CI runs on PRs
• Backend npm test passes (uses mongodb-memory-server)
• Frontend npm run build passes

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[Copilot]

Pull request overview

Adds a GitHub Actions CI workflow and adjusts backend test scaffolding so the backend test suite and frontend build can run reliably in CI (and locally without external dependencies).

Changes:

• Adds a GitHub Actions workflow to install dependencies, run backend tests, and build the frontend on pushes/PRs to main.
• Updates backend test setup to provide JWT secrets during test runs and removes a stale route import/registration from the test Express app.
• Fixes a minor formatting issue in the backend .env.example.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
backend/tests/utils/testApp.js	Removes stale Wikimedia OAuth route wiring from the test app.
backend/tests/setup.js	Sets test-only JWT secrets during test environment setup to prevent auth-related failures.
backend/.env.example	Minor formatting/line alignment fix.
.github/workflows/ci.yml	Introduces CI workflow to run backend tests and frontend build.

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