fix(launchbox): import local media when matching via update_rom

[gantoine]

Description
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Manually (re)matching a LaunchBox ID through the ROM edit dialog ("Update metadata") never imported any LaunchBox media (e.g. local videos), even with video in scan.media. Unlike the ScreenScraper block in the same handler, the LaunchBox branch of update_rom:

• fetched metadata via get_rom_by_id without fs_name/platform_slug, so local-media merging never ran;
• never called populate_rom_specific_paths, so no {type}_path (e.g. video_path) was generated;
• had no store_media_file loop, so nothing was ever copied into the resource store.

This PR fixes the LaunchBox branch of update_rom to:

1. Pass fs_name/platform_slug to get_rom_by_id so local LaunchBox media resolves.
2. Call populate_rom_specific_paths on the fetched metadata to generate media paths.
3. Add a store_media_file loop mirroring the ScreenScraper/scan blocks, which also clears stale media when the LaunchBox ID changes.

Note: the scan/rescan path already imports LaunchBox video correctly (populate_rom_specific_paths is called in scan_handler.py with a persisted ROM, and the scan socket store loop copies it). The gallery-art (non-video images) question raised in the issue is intentionally out of scope here, as it needs a product/UI decision plus a new frontend surface.

AI assistance disclosure: This change was written with AI assistance (PostHog Code / Claude). The investigation, code changes, and tests were AI-generated and reviewed before submission.

Fixes #3521

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)

N/A (backend-only change)

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Created with PostHog Code

[Copilot]

Pull request overview

Fixes a gap in the update_rom flow where manually setting/changing a LaunchBox ID (“Update metadata” from the ROM edit dialog) would fetch LaunchBox metadata but never resolve/copy local LaunchBox media (notably videos) into RomM’s resource store.

Changes:

• Pass fs_name and platform_slug to LaunchBoxHandler.get_rom_by_id so local LaunchBox media can resolve during manual (re)match.
• Run populate_rom_specific_paths for LaunchBox metadata during update_rom to generate {type}_path entries (e.g. video_path).
• Add a LaunchBox store_media_file loop (including stale-media cleanup on ID change) mirroring the existing ScreenScraper logic, and add an endpoint test covering local video import.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backend/endpoints/roms/init.py	Ensures LaunchBox manual rematch resolves local media paths and copies media into the resource store (with cleanup when LaunchBox ID changes).
backend/tests/endpoints/roms/test_rom.py	Adds a regression test asserting LaunchBox local video is assigned a video_path and copied via store_media_file during update_rom.

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