Make the library loader sloppak/loose-only

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).

tests/test_settings_export.py

@@ -583,8 +583,6 @@ def test_partial_field_uses_relpaths_not_absolute(client, server_mod, tmp_path,
     ({"demucs_server_url": 42}, "demucs_server_url"),
     ({"default_arrangement": ["Lead"]}, "default_arrangement"),
     ({"dlc_dir": 42}, "dlc_dir"),
-    ({"psarc_platform": "windows"}, "psarc_platform"),
-    ({"psarc_platform": 42}, "psarc_platform"),
 ])
 def test_import_refuses_invalid_server_config(client, tmp_path, bad_cfg, needle):
     """Importer must apply the same per-key type/range gates that
@@ -627,28 +625,6 @@ def test_import_passes_through_unknown_server_config_keys(client, tmp_path):
     assert cfg["master_difficulty"] == 60
 
 
-def test_import_allows_psarc_platform_null(client, tmp_path):
-    """psarc_platform: null passes validation and is written verbatim
-    (import is full-replace, not partial-update). The server treats a
-    null on-disk value the same as 'both' via the .get(..., 'both')
-    fallback in _background_scan."""
-    (tmp_path / "config.json").write_text(json.dumps({"psarc_platform": "pc"}))
-
-    bundle = {
-        "schema": 1,
-        "server_config": {"psarc_platform": None},
-        "plugin_server_configs": {},
-    }
-    r = client.post("/api/settings/import", json=bundle)
-    assert r.status_code == 200, r.json()
-    # Import is full-replace: null is written verbatim; validate the
-    # disk state so a future regression (e.g. null erroneously rejected
-    # or silently dropped) is caught immediately.
-    on_disk = json.loads((tmp_path / "config.json").read_text())
-    assert "psarc_platform" in on_disk
-    assert on_disk["psarc_platform"] is None
-
-
 # ── Export refuses to follow symlinked subdirectories ───────────────────────
 
 def test_export_skips_symlinked_subdir_contents(client, server_mod, tmp_path):