Remove package-lock.json and add non-yarn lock files to .gitignore

[trillium]

Overview

backend/package-lock.json was accidentally introduced during the ESM migration (PR #2134). The project uses Yarn, so this file should be removed and non-yarn lock files should be gitignored to prevent recurrence.

Action Items

• Remove backend/package-lock.json
• Add package-lock.json and pnpm-lock.yaml to .gitignore
• Follow up with Add pre-commit hook to reject lock files from non-project package managers #2139 to add a pre-commit hook that rejects non-yarn lock files at commit time

Resources/Instructions

• PR Remove package-lock.json and gitignore non-yarn lock files #2140 (implementation)
• PR Migrate backend from CommonJS to ES Modules #2134 (ESM migration that introduced the file)
• Issue Add pre-commit hook to reject lock files from non-project package managers #2139 (pre-commit hook for ongoing prevention)