The frontend commits edited cell values as plain strings (and PK locators as
numbers/booleans), but tokio-postgres' String binds only text-ish types and
i64 only int8, so editing integer/boolean/uuid/jsonb/… columns failed with
'cannot convert between the Rust type and the Postgres type'.
Introduce PgText — a ToSql impl that accepts any Postgres type and writes the
value as UTF-8 in text wire format so the server parses it into the column's
own type. NULL binds as SQL NULL. Adds the bytes crate and unit tests.