Previously the endpoint only accepted `library=<id>` (single id) — multi-
select scopes had to be filtered upstream by Apollo, which kept the
filter logic out of FileViewer-React's reach (it calls ImageApi
directly and got no scoping for 2+ active libraries).
Adds `library_ids` (comma-separated id list, e.g. `?library_ids=1,3`).
Parsed inside the existing scope decision: `library_ids` wins when
both are supplied; either / both empty falls back to "every enabled
library" (historical default). Malformed entries return 400.
Dedupes ids while preserving order so a stray `library_ids=1,1,3`
doesn't double-pass to the DAO. The single-id path still works
unchanged for older clients.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
