SQLite to Postgres migration aborted with "copy *model.ClientInbound:
primary key required" on installs whose client_inbounds table exceeds
one read batch (500 rows). gorm's FindInBatches pages between batches
using a single PrioritizedPrimaryField, which composite-key tables
(client_id + inbound_id, no surrogate id) do not have, so it returns
ErrPrimaryKeyRequired once a table holds more than one batch.
Replace FindInBatches in copyTable with explicit LIMIT/OFFSET paging
ordered by the model's primary-key columns. This works for every table
including composite-key ones, keeps memory bounded, and changes no
schema.
Add a Postgres-gated regression test covering a >500-row composite-key
table.
