refactor: replace hardcoded MongoDB URLs with process.env.MONGO_URL

[Geethegreat]

Issue:

Fixes #

This is a cleanup task identified during E2E testing infrastructure work.

Demo:

Changes:

Replaced hardcoded mongodb://localhost:27017/p5js-web-editor connection strings in migration scripts with process.env.MONGO_URL. No behaviour change for existing setups.

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• is descriptively named and links to an issue number, i.e. Fixes #123
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