Remove the vestigial encrypted-archive ingestion path that the loader can
no longer service:
- scanner no longer lists .psarc files; library is sloppaks + loose folders
- upload endpoint accepts .sloppak only (drop .psarc + magic-byte branch)
- delete endpoint handles sloppak/loose only
- drop the psarc_platform (pc/mac) setting, its validation and tests
- default CONFIG_DIR no longer references an external game data folder
- drop the now-unused pycryptodome dependency (no module imports it)
- correct stale comments that described scanning as archive decryption
Updates test_settings_api / test_settings_export to match (scan fixtures
now build .sloppak stubs).
