Replace Hitster references with Music Cards branding

Updated all references from "Hitster-style" to generic "Music Cards"
throughout the codebase and documentation. This removes brand-specific
terminology and provides a more neutral description of the tool's purpose.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

generate_hitster_cards.py

@@ -276,7 +276,7 @@ def generate_pdf_interleaved(cards: List[TrackCard], out_dir: str, show_year_on_
     print(f"Generated interleaved PDF: {os.path.abspath(out_path)}")
 
 def main():
-    parser = argparse.ArgumentParser(description="Generate DIN A4 Hitster-style cards from a Spotify playlist.")
+    parser = argparse.ArgumentParser(description="Generate DIN A4 music cards from a Spotify playlist.")
     parser.add_argument("--playlist", required=True, help="Spotify playlist URL (public).")
     parser.add_argument("--client-id", required=True, help="Spotify CLIENT_ID.")
     parser.add_argument("--client-secret", required=True, help="Spotify CLIENT_SECRET.")