Expand SoundCloud downloading functionality

[Electr0Fi]

All of the functionality below has been "Vibe Coded" using ChatGPT.

• Modifed pkg/builder/url.go to recognize and accept SoundCloud user profile URLs (e.g., https://soundcloud.com/username) as valid feed sources
• Distinguishes between playlist URLs (/sets/) and user profile URLs (just /)
• Sets appropriate Feed.LinkType (likely TypeUser) for profile URLs
• Implemented SoundCloud API integration to resolve usernames to user IDs and fetch user track lists
• Converts fetched tracks into Podsync episode entries (title, audio URL, duration, cover art, publication date)
• Extended existing playlist handling logic to work with user profile feeds
• Added configuration support for SoundCloud API client ID under [tokens] in config and PODSYNC_SOUNDCLOUD_CLIENT_ID environment variable
• Ensured the solution works with the existing soundcloud-api Go library's client ID scraping mechanism

NOTE: Requires testing before merge!