feat(converto): backend integration for rom conversion and metadata extraction

[DevYukine]

Description
^{Explain the changes or enhancements you are proposing with this pull request.}

This is a draft PR. The core is in, but more will land here before it is ready to merge.

This adds an optional rom-converto backend integration, off by default and gated behind ROM_CONVERTO_ENABLED.

When enabled, scans run rom-converto info on supported platforms (3DS, NDS, PSP, Vita, PS1, PS2, PS3, GameCube, Wii, Wii U, Switch, Dreamcast, Saturn, Sega CD, Xbox, Xbox 360) and store the extracted title id, serial and version in new title_id columns on Rom and RomFile. The columns match the sigil branch schema so both can converge. During the scan, rom-converto runs before sigil: converto writes the title ids it finds, and sigil then only fills what converto left unset, trusting its values. Save-target guidance stays sigil's job.

Admins can also configure per-platform target formats in the new convertto config section. When that is set and download conversion is enabled, single-file downloads are served in the configured format (CIA decrypted, CHD, RVZ, ...) through a disk cache with TTL cleanup. Anything that fails, including oversized files, falls back to serving the original rom. A manual "Convert library" task can pre-warm the cache.

The Dockerfile installs a pinned, checksum-verified rom-converto v0.21.0 binary.

Disclosure
I am the author of rom-converto. I used GLM 5.3 for this but verified and reviewed the result before submitting.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

Backend change only, no screenshots needed