The SQLCipher engine used a dummy sqlite:// URL with a creator function,
which caused SQLAlchemy to auto-select SingletonThreadPool. This pool
non-deterministically closes in-use connections when thread count exceeds
pool_size (default 5), leading to use-after-free segfaults (exit code 139)
in the native sqlcipher3 C library during multi-threaded operations like
user signup.
Now defaults to NullPool (each operation creates/closes its own connection)
for maximum safety with the native C extension. Also respects the
DATABASE_POOL_SIZE setting: if explicitly set >0, QueuePool is used with
the configured pool parameters, matching the behavior of other DB paths.
Fixes#22258
Classic298
04fae8b357