refactor(settings): replace the TMDB tab with settings for every metadata source

[revam]

Why

The server manages every metadata source and its providers through /api/v3/Metadata/Provider and /api/v3/Metadata/Source/{source}/Providers, and moved TMDB's settings out of the core settings. The TMDB tab read core settings that are gone.

What changed

• Settings: the TMDB tab becomes Metadata (/settings/metadata), each source with its icon and status, a switch per kind or a provider order (active and standby), auto-linking, and each provider's own Configuration inline (PUT /api/v3/Configuration/{id}). The kinds and their orders always come from GET …/Source/{source}/Providers, which 6.0.0-dev.495 has.
• The provider changes are a draft saved with the settings on Save and dropped by Cancel; a failing part stays with the server's error.
• Images: Image.MetadataSourceDefaults with per-source overrides replace TMDB's image settings; Metadata purge days added.
• Link export and import for any source through /api/v3/Metadata/{source}/CrossReferences/Export and …/Import, replacing Tmdb/Export and Tmdb/Import.
• First run: the TMDB tab becomes a Metadata tab with the page's search option, image options and sources, through the components the page uses (MetadataSearchSettings, MetadataSourceList); its provider changes are a draft sent with the step's settings.
• The core TMDB settings types and components are removed.

Where to look first

• src/core/react-query/metadata/draft.ts: how provider changes wait for Save and what is sent.
• src/pages/settings/tabs/MetadataSettings.tsx: the page, and src/components/Settings/MetadataSitesSettings/MetadataSourceList.tsx, its sources, shared with the first run.
• src/pages/settings/SettingsPage.tsx: saving the settings and the draft together.

Feedback wanted

Whether saving provider changes with the settings' Save button is the right model, and the provider order UX.

Stack

Part 12 of 12 in the split of #1477. Below: Part 11, site links, source icons and the source picker. Above: nothing; this is the top. Its tree is #1477's plus the review fixes.

Testing

pnpm tscheck, pnpm lint, pnpm test and pnpm build pass at this branch's tip. Manual, on 6.0.0-dev.495: reorder providers and toggle auto-linking, save and cancel; export TMDB links and import them back; run the first-run Metadata tab.

Title and description sources

Settings > Collection now chooses which sources titles and descriptions come from, and in what order. These are the server's Language.SeriesTitleSourceOrder, EpisodeTitleSourceOrder and DescriptionSourceOrder. Until now the WebUI had no way to edit them. With a description order of [TMDB, AniDB], for example, an overview that only TVDB has is never shown.

• The language orders are renamed Series Title Language, Episode Title Language and Description Language. Each one is now followed by a … Source list built like them: drag to reorder, - to remove and + to add.
• The + picker lists AniDB and every source from GET Metadata/Source (TMDb and the plugin sources, such as TvDB and AniList). Each source has its icon (Metadata/Source/{source}/Icon, with a generic icon when there is none) and its name, plus the plugin's name in parentheses when it differs.
• A source name in the settings that no route lists, such as one from a plugin that isn't loaded, stays in the list under its own name and is saved back unchanged.
• User texts aren't offered, because they are implicit: the server always uses them after the listed sources, and a user's pick wins whatever the order. A line under the lists says so. A user entry already in a saved order is kept as a plain entry that can be removed.
• Changes go into the settings draft and are sent by the page's Save, as a JSON patch on Language.*SourceOrder.
• The list part of LanguageOrderList is now a shared OrderList, so the language lists look and behave the same as before.

Testing

1. Open Settings > Collection. Each language order should be followed by its source order, showing the current values with their icons.
2. Click + on Description Source, tick TvDB (or any plugin source) and click Save in the picker. The source is added to the end of the list, and the unsaved-changes toast appears.
3. Drag it above TMDb, then remove one source with -. Cancel should restore the saved order.
4. Save the page and reload. The order should persist in settings-server.json under Language. The orders take effect after a server restart, because they are [RequiresRestart] on the server.

Screenshots

Settings overview: Metadata

Settings overview: Collection

Language Options: title and description sources

Source picker

Metadata: General Options

Metadata: Image Options

Metadata: TMDb source

Metadata: TvDB source

Metadata: AniList source

Metadata: TMDb providers

Metadata: TvDB providers

Metadata: AniList providers

Metadata: Cross-References

Dialog: Image Languages

Dialog: Export cross-references

Dialog: Import cross-references