fix: S3Client singleton and R2 error logging

From the PR description

Summary

• Hoists S3Client to a module-level singleton (lazy-initialised on first use) - prevents a new client being allocated on every R2 operation and enables HTTP keep-alive connection reuse
• Adds console.error logging to the catch blocks in downloadFile and getSignedUrl so R2 outages appear in server logs rather than silently returning null
• Removes stray editResolutionLogging.test.ts that was copied from an unrelated branch and caused test failures on this branch

Closes #101 Closes #102 Closes #115 Closes #116 Closes #125

Changes

• backend/src/lib/storage.ts - singleton _client + error logging in catch blocks
• backend/src/lib/__tests__/storage.test.ts - basic storage module tests
• backend/vitest.config.ts - isolate: true added for consistent module isolation
• backend/package.json - "test": "vitest run" script added

Test plan

• Unit tests added and passing (3/3)
• Backend build passes