Add extensible notification providers + UI/API

Introduce a provider-based notification system and wire it through the API and admin UI. Added INotificationProvider + notification service implementation and providers (apprise, discord, ntfy, pushover), plus a GET /api/admin/notifications/providers endpoint to expose provider metadata. Refactored code to use provider type strings (removed enum coupling), updated masking/encryption calls, and simplified the test notification endpoint to accept backendId or type+config and call sendToBackend directly.

UI: NotificationsTab now fetches provider metadata and renders provider cards and dynamic config forms (fields driven by provider metadata). Added config field rendering, improved backend cards, and edit/delete actions.

APIs: New providers route, updated admin notification CRUD routes to validate provider types dynamically, updated test route schema. Added download-client categories POST API to fetch categories from clients and wired postImportCategory handling in download-client routes.

Other notable changes: BookDate now fetches Claude models dynamically from Anthropic's Models API; added paginated model fetch helper. Added ALLOW_WEAK_PASSWORD flag exposure to auth providers and password change logic. Doc updates and various tests added/updated. File-organization doc clarifies EPERM fix using stream-based copy.

documentation/archive/BOOKDATE_COMPLETE.md

@@ -75,7 +75,7 @@ docker-compose logs -f app
 ## 📊 Feature Highlights
 
 ### AI-Powered Recommendations
-- **Providers:** OpenAI (GPT-4o+) or Claude (Sonnet 4.5, Opus 4, Haiku)
+- **Providers:** OpenAI (GPT-4+) or Claude (dynamically fetched from Anthropic Models API)
 - **Personalization:** Based on your Plex library + swipe history
 - **Context:** Max 50 books (40 library + 10 swipes)
 - **Filtering:** Excludes books already in library, already requested, or already swiped