chore: add lint script and enforce linting in CI

[BryanFRD]

Package
Which package does this affect? [x] server [ ] tools [ ] docs

Problem / Motivation
CONTRIBUTING.md references `pnpm lint` but no `lint` script exists in `package.json`. CI also doesn't run linting.

Proposed solution
To be defined — see Problem / Motivation above.

Alternatives considered
None considered explicitly — this is the natural shape.

Additional context
Fix

• Add a `lint` script to `package.json` (e.g., `eslint src/`)
• Add ESLint config
• Add lint step to `.github/workflows/ci.yml`