A CSV saved as UTF-16 (Excel's "Unicode Text") was read as UTF-8, leaving
a NUL between every character. MySQL and SQLite store that, Postgres text
columns reject it with SQLSTATE 22021 and the whole import fails.
The file is now decoded to UTF-8 when read: a UTF-16 BOM switches to
UTF-16 decoding, remaining NUL bytes are dropped and invalid UTF-8 becomes
U+FFFD, matching what encoding/json does for the JSON importers.
Fixes API-CLOUD-4G
