Port from qwibitai/nanoclaw#3766: hold the SQLite write lock across a
schema migration so two openers cannot interleave.
The SessionDB writer connection is autocommit, so _rebuild_table's
RENAME / CREATE / copy / DROP each committed on its own. A sibling
process (gateway + CLI opening the same state.db) whose SCHEMA_SQL
bootstrap ran between RENAME and CREATE recreated the table empty; the
rebuilder's CREATE then failed with "table already exists", the heal
was logged as "skipped", the rows stayed in *_legacy_pk and the live
table was empty — silent loss of routing / usage accounting rows.
BEGIN IMMEDIATE around the whole sequence (unless the caller already
owns a transaction) makes the sibling wait under its existing lock
patience, and a mid-rebuild failure rolls the RENAME back.
teknium1
caa7f21f4c