Fix storage maintenance retention cleanup

[Rabithua]

Summary

• bind the 30-day reservation retention cutoff as an ISO timestamp instead of a raw JavaScript Date
• increase official baseline progress from 10 to 100 users per run while keeping object-store HEAD concurrency bounded at 8

Root cause

The untyped raw SQL fragment passed a Date directly to the Postgres driver, which rejected it after every maintenance run.

Validation

• resource worker tests: 4 passed
• Server TypeScript build
• changed-file ESLint
• git diff check