Some target versions add a ``UNIQUE`` constraint on a field that allowed
duplicates in the source version (e.g. ``utm.source.name`` in 16.0). The
unique index creation then aborts the whole registry load with a
``psycopg2.errors.UniqueViolation``.
Declare such constraints per version in ``known_changes.yaml`` under the
``new_unique_constraints`` key (only ``model`` and ``fields`` needed).
During ``prepare_db.sh``, ``dedup_unique_constraints.py`` aggregates the
declarations of every traversed version and, on the source database,
renames duplicates by suffixing `` [<id>]`` while preserving foreign keys.
The actual PostgreSQL column type is used (not the ORM ``translate``
attribute) so both ``varchar`` and already-converted ``jsonb`` columns are
handled: ``jsonb`` duplicates are compared and renamed on every language
key (indexed ``en_US`` value). Renamed records are reported in a
timestamped JSON file and summarized at the end of ``migration.log``.
