Feature/song service

[myndaaa]

• removal of exiting song upload and stream functions which used to return just the file path or POST a song entity.
• added missing album and albumsong data models crud functions and endpoints.
• updates in backend/app/crud/artist.py and backend/app/schemas/artist.py for consistency with album/song usage.
• file service, song service schemas and functionalities implemented to assist in uploading of images and songs, and rendering of images and streaming of songs
• for song service, used mutagen library to extract audio metadata

Logic Implemented:

• On upload, file is temporarily stored, validated then metadata is extracted. Once done a database record created with empty file_path, then file is moved to permanent location with unique filename format: {type}{id}{uuid}.{ext}), and database updated with actual file path.
• For streaming /stream/song/{song_id} checks database for song existence, validates file exists on disk, then handles HTTP Range headers for seeking, parses "bytes=start-end" format and validates range bounds, then seeks to start position in file & reads only requested chunk [returns 206 Partial Content]