[Bug] M3U multi-file game sizes not included in platform storage stats

[yamanobe96]

Describe the bug
When a platform uses .m3u files to manage multi-disc games, the storage size shown in the server stats page only reflects the size of the .m3u text files themselves, not the actual ROM files (ISO, CHD, BIN, etc.) referenced within them.
To Reproduce
Set up a platform (e.g. PS2) where games are stored as .m3u + referenced ROM files in a subdirectory
Scan the platform
Check the server stats page → platform size is only a few MB despite containing hundreds of GB of ROMs
Expected behavior
The platform storage size should include the total size of all files referenced by .m3u files, not just the .m3u files themselves.
Additional context
Platform: PlayStation 2 (2484 games)
Displayed size: 12.06 MB (only .m3u files)
Actual size on disk: several hundred GB
ROM files are stored in subdirectories relative to each .m3u file

[yamanobe96]

Related issue: M3U files referenced by the same structure also fail to resolve disc selection during playback. The root cause appears to be the same — RomM treats .m3u as a flat file rather than parsing its contents to discover referenced ROMs. A separate issue will be filed for the playback behavior.

[yamanobe96]

Update: size accounting is also fixed by the same workaround

I confirmed that when the M3U parsing patch described in #3363 is applied,
the referenced CHD/ISO files are registered as RomFile entries in the DB,
and their sizes are included in the platform storage stats.

Before patch: 12.06 MB (only .m3u text files)
After patch + Complete Scan: correct total reflecting all CHD files on disk

So both issues (#3360 and #3363) share the same root cause and the same fix:
parsing .m3u contents in get_rom_files() and registering the referenced
files as RomFile entries rather than treating the .m3u as a single opaque file.