Add Dependabot version-update cooldown

[ld-repository-standards]

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• Ensure every entry under updates in .github/dependabot.yml declares a cooldown of at least 7 days (default-days).
• Add entries for detected package ecosystems that were not yet tracked by Dependabot.

Cooldown applies only to version updates; security updates bypass it, so critical CVE fixes are never delayed.

Ref: SEC-8058.

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Note

Low Risk
CI/dependency automation only; no application runtime or auth logic changes, and security updates bypass the cooldown.

Overview
Introduces Dependabot via a new .github/dependabot.yml so GitHub Actions and Gradle dependencies get weekly version-update PRs.

Each ecosystem sets a 7-day cooldown (default-days: 7) on version bumps to reduce update noise; security updates are not delayed by this cooldown.

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