Fix backup script execution order and add post-upload verification

[Peolite001]

Summary

Fixes the database backup script execution order to prevent silent backup failures. Previously, the script attempted to call upload_to_s3 and upload_to_gcs within the case statement before those functions were actually defined. Due to set -euo pipefail, this caused the script to exit with a "command not found" error immediately after a successful local dump, meaning backups were never uploaded.

Key Changes:

• Restructured scripts/backup-db.sh to use a standard main() function invoked at the bottom, ensuring all functions are defined before execution.
• Added post-upload verification steps (aws s3api head-object for S3 and gsutil ls for GCS) to verify the integrity of the uploaded backup, surfacing a distinct error if the upload fails to land in the bucket.

Type

• Bug fix
• New feature
• Documentation
• Refactor
• Smart contract change

Related Issue

Closes #157

Testing

• Tested locally on Testnet
• No TypeScript / Rust errors
• Docs updated if needed

Infra / Docs Sync

• If this PR changes infrastructure (k8s/ manifests, workflows, scripts), docs/* claims were verified against the manifests (deployment, database, indexer, runbooks)

Screenshots (if UI change)

N/A