ci: add GitHub Actions workflow for build and unit tests

[QuigProQuo]

What

Adds a GitHub Actions CI workflow (.github/workflows/ci.yml) that runs on every push and PR to the dev branch.

Why

The repository currently has no CI/CD pipeline at all — no GitHub Actions, no Travis, no Jenkins. This means:

• Builds and tests are only run if a developer remembers to do so locally
• Broken code can reach dev without detection
• There's no automated signal for PR reviewers

This PR adds the foundational CI step: build + unit tests on every push/PR.

What It Does

Step	Action	Why
Checkout	actions/checkout@v4	Get the code
Set up JDK 21	actions/setup-java@v4 (Temurin 21, Maven cache)	Matches pom.xml <release>21</release>
Set up Node.js 20	actions/setup-node@v4 (npm cache)	Frontend assets built via npm ci during Maven generate-sources
Compile	mvn compile -B	Verifies Java + webpack build succeeds
Unit tests	mvn test -B	Runs Surefire (@UnitTest group — currently 356 tests, 0 failures)

What It Doesn't Do (Yet)

This is intentionally a minimal first step. Future improvements:

• Integration tests (mvn verify) — requires PostgreSQL + Elasticsearch service containers; should be a separate job
• Flyway migration validation — requires a PostgreSQL service container
• Dependency scanning — can be added via Dependabot or OWASP Dependency-Check
• Security scanning — CodeQL or Semgrep

These can be added incrementally once this basic workflow is merged.

Verification

I ran mvn compile and mvn test locally before submitting this PR:

• mvn compile — BUILD SUCCESS (Java + webpack frontend)
• mvn test — BUILD SUCCESS, 356 tests, 0 failures, 0 errors, 5 skipped