chore: add Dependabot update schedule

[saurabhhhcodes]

Summary

• add Dependabot configuration for root Python, backend Python, frontend npm, and GitHub Actions manifests
• schedule weekly update checks against the dev branch with small grouped update PRs
• add dependency-focused labels and pull request limits per ecosystem

Validation

• ruby -e "require 'yaml'; data = YAML.load_file('.github/dependabot.yml'); abort('missing updates') unless data['updates'].size == 4; puts 'dependabot yaml ok'"\n- git diff --check\n\nCloses devops: Add Dependabot configuration for automated dependency updates #57