Add npm dependency caching to CI

[james2177]

Description

.github/workflows/ci.yml's actions/setup-node@v4 step doesn't set cache: npm, so every CI run does a full npm install from scratch instead of restoring from a cache keyed on package-lock.json.

Requirements and context

• One-line-ish fix, but meaningfully reduces CI run time on every single push/PR given the number of workflow runs this repo already has

Suggested execution

1. Fork the repo and create a branch

   git checkout -b ci/add-npm-cache
   

2. Implement changes
   Relevant files:

• .github/workflows/ci.yml

3. Test and commit

   • Run npm run lint && npm run typecheck && npm test && npm run test:e2e
   • Include test output / screenshots or logs demonstrating the fix in the PR

   Example commit message:

   ci: cache npm dependencies in the CI workflow
   

Guidelines

• Assignment required before starting — comment to claim this issue
• PR description must include: Closes #[this issue's number]
• Timeframe: 96 hours from assignment
• Complexity: High (200 points)

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Part of the Drips Wave for vortex-backend.