Add custom AI provider support and improve qBittorrent auth

Introduces support for custom OpenAI-compatible AI providers with configurable base URLs, including UI, backend validation, and connection testing. Enhances qBittorrent integration to support HTTP Basic Auth for reverse proxies, adds detailed debug logging, and updates documentation for both features. Also improves login page description logic and AI prompt generation for recommendations.

documentation/backend/services/auth.md

@@ -216,6 +216,8 @@ Automatically grants admin permissions based on OIDC claims (e.g., group members
 - **GET /api/auth/oidc/login** - Initiate OIDC flow, redirect to provider
 - **GET /api/auth/oidc/callback** - Handle OAuth callback, create/update user, return JWT
 - **GET /api/auth/providers** - List enabled auth providers for login page
+  - Returns: `backendMode`, `providers[]`, `registrationEnabled`, `hasLocalUsers`, `oidcProviderName`, `localLoginDisabled`, `automationEnabled`
+  - `automationEnabled`: true if Prowlarr/indexer configured (used for dynamic login page description)
 
 ### Configuration Keys
 ```