* feat(checkpoint): make delta snapshot_frequency configurable
* fix(config): carry legacy checkpoint_delta_snapshot_frequency with warning
Addresses review on #4516: the rename from the flat
database.checkpoint_delta_snapshot_frequency key to nested
database.checkpoint_delta.snapshot_frequency silently dropped the old
value (pydantic extra="ignore"). Add a before-validator that maps the
legacy key onto the nested one with a deprecation warning (nested key
wins when both are set), plus a CHANGELOG breaking-change note covering
the rename and the 1000 -> 10 default change.
* fix(checkpoint): validate frozen snapshot frequency