org_slug parameter is case-sensitive with no indication

## Description
The `org_slug` parameter on `/search` is case-sensitive, returning zero results for uppercase values without any error or indication.

## Steps to Reproduce
GET https://catalog.data.gov/search?org_slug=nasa (returns results)
GET https://catalog.data.gov/search?org_slug=NASA (returns zero results)

## Expected Behavior
Either case-insensitive matching, or a clear error when the slug is not found.

## Actual Behavior
Uppercase slug returns zero results silently -- indistinguishable from a valid slug with no datasets.

## Notes
Confirmed April 27, 2026. Slugs are lowercase by convention but this is not documented anywhere. Minimum fix is a documentation note. Preferred fix is case-insensitive matching.