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/phase3/qbittorrent.md

@@ -181,6 +181,13 @@ type TorrentState = 'downloading' | 'uploading' | 'stalledDL' |
    - Headers set to qBittorrent base URL (e.g., `https://seedbox.example.com:443/qbittorrent`)
    - Applied to both `login()` and `testConnectionWithCredentials()` methods
    - Works with all qBittorrent versions and configurations
+   - Enhanced debug logging for troubleshooting authentication issues (enable with `LOG_LEVEL=debug`)
+**13. Nginx/Apache reverse proxy HTTP Basic Auth** - Many seedboxes use nginx or Apache reverse proxy with HTTP Basic Authentication in front of qBittorrent. This causes HTTP 401 errors with `www-authenticate: Basic` header. Browsers handle this by prompting for credentials and sending `Authorization: Basic` header. Fixed by:
+   - Adding HTTP Basic Auth to all axios requests using `auth` parameter
+   - Same credentials used for both Basic Auth (nginx/Apache) and qBittorrent Web UI authentication
+   - Applied to axios client instance and all standalone requests
+   - Works transparently with or without reverse proxy
+   - Compatible with popular seedbox providers (seedit4.me, etc.)
 
 ## Tech Stack