Add `Dependabot` configuration for npm and Cargo

[ritik4ever]

Problem

Dependencies are never automatically updated. Known vulnerabilities may go undetected.

Scope

Add .github/dependabot.yml for weekly updates across all ecosystems.

Acceptance criteria

• npm: weekly updates for backend/ and frontend/
• Cargo: weekly updates for contracts/
• Minor/patch updates grouped into one PR per ecosystem
• PRs auto-assigned to maintainer

Complexity: Easy · 50 points

[espcris05-commits]

/attempt

[Cjay-Cyber-2]

@Cjay-Cyber-2 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

please assign this to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Cjay-Cyber-2 to this issue.

[samueloyibodevv]

@samueloyibodevv has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Based on your request, I will implement and fix this issue according to the design requirement and to fit your acceptance criteria

ℹ️ Repo Maintainers: To accept this application, review their application or assign @samueloyibodevv to this issue.