Fixed the Berkeley-to-Mesa archive migration shipped on `compatible`. `upgrade_to_mesa.sql` now drops the `element_ids` UNIQUE constraint and btree index on `zkapp_events` and `zkapp_field_array` (a btree key over the unbounded `int[]` exceeds Postgres' 2704-byte limit and blocked block inserts for max-cost zkApps), and makes `zkapp_account_update_body.events_id` and `actions_id` nullable so an empty events/actions list is stored as `NULL`. Without these, a migrated archive reported `applied` but could not ingest blocks containing zkApp commands. Archive operators should re-run the migration from an image built after this change.
