Add --defer-validate-fks to create FK constraints as NOT VALID

[teknogeek0]

Problem

For large migrations, foreign-key validation can take hours or even days. Because FK creation happens before the transition into CDC, that validation scan blocks the migration from reaching streaming mode — and while it runs, the replication slot accumulates a large WAL backlog that single-threaded CDC replay must then chew through, delaying completion further.

Solution

Add --defer-validate-fks. When set, pgcopydb creates the target foreign keys as NOT VALID, skipping the one-time validation scan of pre-existing rows so the COPY → CDC handoff happens immediately. The operator validates the constraints later, on their own schedule, with ALTER TABLE ... VALIDATE CONSTRAINT.

This is safe for CDC: NOT VALID still enforces referential integrity on every new write — it only skips the historical-row scan. Any change replayed from the source already satisfied the source's constraint, so it satisfies the target's NOT VALID constraint too. FK creation still completes before CDC apply is enabled, so enforcement is in place from the first replayed change.

Hardening for the common online-migration flag combination

This was reviewed and tested specifically for clone --follow --defer-indexes --defer-validate-fks --split-tables-larger-than:

• Resume consistency: the flag is persisted in the catalog setup table and a resume that changes it is rejected — preventing a resume from silently creating the remaining FKs VALID and triggering the scan the flag exists to avoid.
• Missing referenced unique constraint (SQLSTATE 42830): e.g. a filtered-out unique index on the referenced table. The FK is skipped and captured in an end-of-run summary instead of aborting the migration.
• Partitioned parents (SQLSTATE 42809): PostgreSQL forbids NOT VALID foreign keys declared on a partitioned table. Previously this would abort the whole migration. Now the parent-level constraint is skipped with a captured warning (per-partition FK rows are still created NOT VALID), so the partition data is still enforced and deferred without blocking.
• Cutover reminder: the deferred-FK validation reminder is re-emitted at follow/cutover completion so it is not buried mid-log.

Tests

• tests/defer-validate-fks — clone path: validated FKs land NOT VALID, source-NOT VALID FKs stay NOT VALID, data fully copied, violating writes still rejected, manual VALIDATE succeeds, and the partitioned-parent constraint is skipped without aborting.
• tests/follow-defer-validate-fks — the full --follow --defer-indexes --defer-validate-fks --split-tables-larger-than combination with DML injected during CDC; asserts all FKs land NOT VALID, data stays consistent after replay, enforcement holds, and deferred validation completes.

Registered in tests/Makefile and the tiered CI workflow. Full local suite (PG16) passes.