* fix(migration): use IF EXISTS when dropping chunk FK constraint
The migration unconditionally dropped memory_units_chunk_fkey, but
depending on the order in which migrations were applied the constraint
may not exist. Use raw SQL with IF EXISTS so the drop is safe regardless.
* fix(migration): make chunk FK add idempotent with DO block
The previous fix only handled the DROP side with IF EXISTS. The ADD side
could still fail with DuplicateObject when the FK already existed on a
schema that was provisioned after the base migration ran.
Wrap the ADD CONSTRAINT in a DO block to catch duplicate_object and
continue, making the migration fully idempotent in both directions.
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