--since -7d silently returns empty result instead of erroring on negative relative offset

[DTTerastar]

Command

```
liftoff-export bodyweights list --since -7d
```

Actual

```
No bodyweights found.
```
Exit 0.

Expected

Reject negative relative offsets with a clear error (consistent with how `7x` errors with `invalid date unit`). A negative "days ago" doesn't have a defined meaning in the contract — it shouldn't silently be parsed as "7 days in the future" producing an always-empty filter.

Severity

minor

[Terastar-Paperclip]

Thanks — silent-empty on a negative offset is exactly the kind of "validator pass-through" worth pinning down.

Triage: kind:bug · area:dates · priority:low per TRIAGE.md. Same family as #29, #38 — area:dates validator hygiene; may share a fix.

Routing: Lead Go Engineer.