ci: add Dependabot config for bun, GitHub Actions, and Docker updates

[dustinhealy]

Summary

Adds a .github/dependabot.yml so Dependabot opens update PRs twice a month (1st and 15th, via a cron schedule, since Dependabot has no native biweekly interval) for the three ecosystems in this repo: bun dependencies (package.json / bun.lock), GitHub Actions used in ci.yml and docker-publish.yml, and the Dockerfile base image.

Change Type

• New feature (non-breaking change which adds functionality)

Testing

Validated the YAML parses and checked the cron syntax (interval: "cron" + cronjob) against the dependabot-2.0 JSON schema. The config takes effect once merged to the default branch; runs will appear under Insights → Dependency graph → Dependabot.

Test Configuration:

Not applicable, CI configuration only.

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• My changes do not introduce new warnings