Add Flathub ecosystem support

[andrew]

Adds support for indexing Flatpak apps from Flathub.

Uses the v2 REST API:

• /api/v2/appstream for the full package list (~3250 apps)
• /api/v2/appstream/{id} for per-app metadata (license, summary, vcs_browser, releases, categories, verification status)
• /api/v2/stats/{id} for total install counts
• /api/v2/collection/recently-updated and recently-added for incremental syncs

The flatpak runtime and sdk are recorded as dependencies on the latest version so reverse lookups like "what runs on org.gnome.Platform" work. purl type is flatpak.

To enable in production:

Registry.find_or_create_by(name: 'flathub.org', url: 'https://flathub.org', ecosystem: 'flathub', github: 'flathub', default: true).sync_all_packages_async

Closes #1614