Add Dependabot configuration for automated dependency updates

[cyber-excel10]

Closes #55

Summary

Adds .github/dependabot.yml to enable automated dependency update PRs across all three package ecosystems in this repo, per #55.

What's included

• npm ecosystem for /api
• npm ecosystem for /oracle
• cargo ecosystem for /stellar-lend
• All three on a weekly update schedule
• Minor and patch updates grouped per ecosystem to reduce PR noise (per the issue's constraints)

Why

The project currently has no automated mechanism to surface dependency updates, including security patches in transitive dependencies. This config lets Dependabot open update PRs automatically so those can be reviewed and merged on a predictable cadence instead of going unnoticed.

Testing

This is a configuration-only change — no source code is modified. I validated the YAML locally before submitting. Once merged into the default branch, GitHub will pick up the config automatically and Dependabot will run its first scheduled scan.

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⚠️ GitGuardian has uncovered 7 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

Since your pull request originates from a forked repository, GitGuardian is not able to associate the secrets uncovered with secret incidents on your GitGuardian dashboard.
Skipping this check run and merging your pull request will create secret incidents on your GitGuardian dashboard.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
34253505	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253506	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253507	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253508	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253509	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253510	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret
34253511	Triggered	Generic High Entropy Secret	bed18b9	docker/devnet/seed-data/accounts.json	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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