feat(cloud-sync): read-only WebDAV browsing (PROPFIND) for the rom library

[fmustafayaman]

Summary

Adds read-only WebDAV directory browsing (PROPFIND, plus a fake LOCK/UNLOCK handshake some clients require) to the same /api/cloud-sync WebDAV surface RetroArch's Cloud Sync uses. This is unrelated to RetroArch's own sync traffic -- verified against its source, RetroArch's Cloud Sync client never issues PROPFIND -- it's purely so a real WebDAV client (iOS Files' "Connect to Server", Cyberduck, ...) can mount the same URL and browse the rom library (and current saves/states) as plain files.

Ports the retroarch-webdav-romm shim's romBrowser.ts + webdavXml.ts design:

• roms/<platform_fs_slug>/<file> -- lists platforms with roms, then roms within a platform, using the same display-name logic as the shim (fs_name_no_ext.zip for multi-file roms, the real per-file name for single-file roms). Read-only: no PUT/DELETE for this tree.
• saves/ / states/ -- browses the current cloud-sync manifest as a virtual directory tree. Unlike the shim, this only shows current entries, not full per-revision history (RomM's own web UI already covers save/state history; re-deriving that here felt out of scope for a browsing convenience).
• LOCK/UNLOCK -- fake, always-succeeds. Some WebDAV clients (iOS Files by report) won't complete "Connect to Server" without a server that at least answers these, even read-only.

For actually downloading a rom file, GET/HEAD redirect (307) to RomM's existing /api/roms/{id}/content/{file_name} endpoint rather than reimplementing Range support, the multi-file zip cache, and (in production) nginx's X-Accel-Redirect -- that endpoint already accepts HTTP Basic Auth alongside OAuth, so the same credentials carry over on the redirect.

Found along the way

Rom.has_multiple_files/.files depend on a deferred column_property and a relationship that get_roms_scalar doesn't eager-load -- accessing them outside that query's own session raised DetachedInstanceError. Fixed by re-fetching the visibility-filtered rom ids via get_roms_by_ids (which does eager-load them via @with_details) instead of using the raw scalar results directly.

Test plan

• 71/71 cloud-sync tests passing (11 new: PROPFIND for root/roms/platform/rom-file/saves-states, LOCK/UNLOCK, GET redirect, 404s, visibility)
• ruff check clean

[fmustafayaman]

Closing in favor of merging this into #3933 instead, to keep everything on one branch.