chore(infra): add Renovate (or Dependabot) for automated dependency updates

[GRACENOBLE]

Problem

There is no automated dependency update strategy. Dependencies across Go, Node, Gradle, and GitHub Actions will go stale silently.

Acceptance criteria

• Add renovate.json (or .github/dependabot.yml) at the repo root
• Configure separate schedules for each ecosystem: Go modules, npm (web), Gradle (mobile), GitHub Actions
• Pin major version bumps to require a PR review
• Group patch/minor updates to reduce PR noise

[GRACENOBLE]

Implemented and merged via PR #66.