[FEATURE] Add Renovate bot for automated dependency updates

**Category:** devops
**Project area:** renovate.json (new)

**Implementation:**
- Configure for npm (frontend, backend) and Cargo.
- Patch updates: weekly batch PRs.
- Major updates: individual PRs.
- Auto-merge patches if CI passes.

**Acceptance Criteria:**
- Renovate PRs open automatically.
- Patches auto-merged after CI.
- Major updates assigned to maintainers.