feat(metadata): add CivArchive API toggle and provider fallback order settings

- Add enable_civarchive_api toggle (default on) to allow disabling
  CivArchive to avoid its rate-limit windows entirely
- Add metadata_provider_order dropdown with two presets:
  CivitAI → CivArchive → Archive DB (default) and
  CivitAI → Archive DB → CivArchive
- Wire both settings through backend (metadata_service, settings_manager,
  misc_handlers) and frontend (SettingsManager, state, settings modal)
- Reorder Metadata section in settings modal: toggles → status/management
  → fallback order, for natural top-down workflow
- Make update_metadata_providers() log the effective provider chain
  using actually-registered providers rather than settings assumptions
- Add 5 test cases covering all provider-combination paths
- Complete i18n translations for 6 new keys across all 9 non-English locales

locales/en.json

@@ -640,7 +640,13 @@
       "preparing": "Preparing download...",
       "connecting": "Connecting to download server...",
       "completed": "Completed",
-      "downloadComplete": "Download completed successfully"
+      "downloadComplete": "Download completed successfully",
+      "enableCivarchiveApi": "Enable CivArchive API as metadata provider",
+      "enableCivarchiveApiHelp": "When on, CivArchive API is used as a fallback source for model metadata (e.g. for models deleted from CivitAI). Turn off to avoid CivArchive rate limits entirely.",
+      "providerOrder": "Metadata provider fallback order",
+      "providerOrderHelp": "CivitAI API is always tried first. Choose the order of the remaining providers when looking up metadata.",
+      "providerOrderCivitaiArchiveSqlite": "CivitAI → CivArchive → Archive DB",
+      "providerOrderCivitaiSqliteArchive": "CivitAI → Archive DB → CivArchive"
     },
     "proxySettings": {
       "enableProxy": "Enable App-level Proxy",