fix(platforms): stop platform variants from renaming the parent platform

[gantoine]

Description
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When a folder is mapped as a Platform Variant (or Folder Alias) of an existing platform, the scanner resolves that folder to the parent's slug. The database then holds two platform rows sharing the same slug (e.g. the real arcade folder and an fbneo folder both with slug="arcade").

get_supported_platforms() built its slug→platform map with a plain dict comprehension:

db_platforms_map = {p.slug: p for p in db_platforms}

With two rows sharing a slug, whichever came last (platforms are ordered by name) silently overwrote the parent, so the platform picker and Folder Mappings page displayed the variant's name instead of the parent's. This is what caused Arcade to be "renamed" to FBneo, and the reported WiiWare/WiiVC case.

The fix prefers the canonical platform (the one whose fs_slug matches its slug) when building the map, so a variant/alias folder can no longer shadow its parent.

Note: variants remain their own platform row borrowing the parent's metadata, which is the documented intent of "Platform variant". This PR only fixes the naming collision.

Fixes #3157

Checklist
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• 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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AI assistance disclosure

This change was written with AI assistance (PostHog Code). The AI performed the investigation, implemented the fix, and wrote the accompanying test; the change has been reviewed before submission.

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Created with PostHog Code

[Copilot]

Pull request overview

Fixes a backend bug where platform variant / folder alias rows that share the parent platform’s slug could silently overwrite the canonical platform entry in get_supported_platforms(), causing the UI to display the variant’s name (e.g., “FBneo”) instead of the parent (“Arcade”).

Changes:

• Update get_supported_platforms() to prefer the canonical platform row (fs_slug == slug) when multiple DB rows share the same slug.
• Add a regression test ensuring a variant row cannot shadow the parent platform’s display name.

Reviewed changes

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

File	Description
backend/utils/platforms.py	Builds the slug→platform map in a way that prevents variant/alias rows from overwriting the canonical platform entry.
backend/tests/utils/test_platforms.py	Regression test covering the “variant shadows parent” slug-collision case.

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