Apply review feedback: use npm ci and remove redundant festivals.json copy

[Copilot]

Addresses two review comments from PR #212 on Worker test infrastructure.

Changes:

• mise.toml: Use npm ci instead of npm install in test:worker task to enforce lockfile-based installs and prevent non-deterministic dependency updates
• .github/workflows/deploy-worker.yml: Remove redundant cp data/festivals.json step—already handled by npm pretest script in package.json

Before:

[tasks."test:worker"]
run = 'npm install && npm test'

After:

[tasks."test:worker"]
run = 'npm ci && npm test'

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