Hoist R2 S3Client to a module-level singleton

The Mike backend was allocating a fresh `S3Client` on every R2 operation. bmersereau's PR hoists it to a module-level singleton with lazy initialization, so the HTTP layer can reuse keep-alive connections instead of reopening sockets per request.

The change is small in scope: one module, its tests, and Vitest config. No behavioral change visible to callers.

The secondary fix is more useful operationally. The download and signed-URL paths previously caught R2 failures and returned null, with no logging. That made object-storage outages invisible from the server logs. Error logging is added to both catch blocks, so failures surface rather than disappear silently.

A stray test file copied from another branch was also removed, since it was breaking the suite here. Vitest is configured for module isolation and a proper test script is wired into the backend package.

The PR closes five tracked issues (#101, #102, #115, #116, #125), which suggests this consolidates a cluster of storage-reliability complaints that had accumulated separately.