MySQL can't index a BLOB/TEXT column without a prefix length, so
create_all() on a fresh MySQL schema blew up with "BLOB/TEXT column
'kd_digraph_simhash' used in key specification without a key length".
SimHashes are a fixed 8 bytes — the variable-length type was a
SQLAlchemy-side auto-mapping from 'Optional[bytes]', not an actual
schema requirement. Switch to BINARY(8), which is portable: MySQL gets
a fixed-width indexable BINARY, SQLite treats it as BLOB and doesn't
care about key length.
anti
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