[legluondunet]

Title: Add website support for Flatpak runner installer scripts

Summary

This PR adds support for Flatpak-based installer scripts to the Lutris website.

It complements the Flatpak runner improvements proposed for the Lutris client.

Changes

• Register the Flatpak runner in the website database.
• Support Flatpak runner selection in installer submission forms.
• Validate Flatpak-specific installer configuration.
• Support Flatpak installer scripts through the existing submission, moderation, and API workflows.
• Add automated tests for forms, validation, runner registration, and HTTP endpoints.

Compatibility

This change is designed to work with the companion Lutris client PR introducing the install_flatpak task.

Example installer configuration:

game:
  appid: technology.haniwa.ai5-sdl2
  args: '--font "MS PGothic.ttf"'

installer:
- task:
    name: install_flatpak
    file: engine

Testing

The website's GitHub Actions CI passed, including the automated test suite.

Related PR

Companion PR: Add Flatpak runner with installer and uninstall support (Lutris client).

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