[Bug]: `max-query-parallelism` accepts values outside documented 1-8 range

The docs state the valid range for `max-query-parallelism` is 1-8. However, both `0` and `9` are accepted without error.

## Expected

DAB should reject values outside the 1-8 range at config deserialization.

## Actual

| Value | Result |
|-------|--------|
| `0` | Accepted, DAB starts, health checks run (9 checks returned) |
| `9` | Accepted, DAB starts, health checks run (9 checks returned) |

**Impact:** `max-query-parallelism=0` is particularly concerning — it's unclear whether this means "no parallelism" (sequential) or "unlimited parallelism". Testing showed health checks still execute and return results, so `0` likely defaults to some internal value.