[Feature] An opt-out for the provider metadata blocks on GET /api/roms #5227

Is your feature request related to a problem? Please describe.

A companion app that mirrors a RomM library onto a device walks the whole library through GET /api/roms?platform_ids=…&limit=500&offset=… on its first sync and on every forced resync. It needs about 25 fields per ROM: the ids, names, fs_size_bytes, platform, regions, languages, revision, tags, rom_user, the two cover paths and five sub-fields of metadatum. Every item nevertheless carries the full provider metadata, and that is where the bytes are.

Measured on RomM 5.3.1, one page of 500 with with_char_index=false&with_filter_values=false:

The existing opt-outs do not reach this: with_rom_id_index=false saves 25 KB per page (0.3 %), with_total=false nothing measurable, and a page of 100 takes ~0.16 s against ~0.9 s for 500, so the cost is per item rather than per request. Over a whole library that is roughly five bytes serialized on the server and parsed on the client for every one the client reads.

Describe the solution you'd like

One more opt-out in the family GET /api/roms already has (with_char_index, with_filter_values, with_rom_id_index, with_total): with_metadata=false (name yours), default true, which leaves the provider blocks off each item — the thirteen *_metadata objects, merged_ra_metadata, merged_screenshots and alternative_names — and, under the same switch or a second one, sibling_roms. metadatum and every flat field stay. Nothing changes for a caller that does not pass the flag, the v2 gallery included. #3781 and #4062 added opt-outs of the same shape for the id index and the count.

Describe alternatives you've considered

• A fields= sparse-fieldset parameter: more general, but a different shape from the opt-outs the endpoint has, and harder to keep in step with the schema.
• GET /api/roms/{id}/simple per ROM: the same SimpleRomSchema, so the same blobs, times the library.
• Smaller pages or fewer requests on the client: measured, no effect, since the cost is per item.