Add Spotify cache as a junk category

[Vortrix5]

Summary

Spotify keeps a large media cache under %LOCALAPPDATA%\Spotify\Storage and %LOCALAPPDATA%\Spotify\Data (cached audio/data, re-downloaded on demand). It's safe to clear and grows a lot for heavy users. Add it as a junk category.

Where

Follow the existing app-cache categories in src/sifty/core/junk.py. Base dir is %LOCALAPPDATA% (use the existing _local_appdata() helper).

Acceptance criteria

• New category shows up in sifty junk scan
• Only the cache/storage dirs, nothing holding login/session data
• Sandbox test in tests/test_junk.py
• pytest green

Tip

The /new-junk-category skill scaffolds the category, its safety carve-out, and a test.