A mere add_foreign_key can wipe out your whole Rails+SQLite production table

We likely should be disabling and re-enabling the foreign_keys PRAGMA before/after the transaction to change the table per the sqlite docs.

Edit: actually it looks like we already do that since 45881b0

Edit2: Ah, this is complex. We currently do

transaction do
  disable_referential_integrity do

but it should be

disable_pragma_foreign_keys do
  transaction do
    enable_defer_foreign_keys do

because PRAGMA foreign_keys

is a no-op within a transaction; foreign key constraint enforcement may only be enabled or disabled when there is no pending BEGIN or SAVEPOINT.

per doc, and "PRAGMA foreign_keys before TRANSACTION" is the order recommended in the first sqlite doc link above. However, PRAGMA defer_foreign_keys is set per transaction so it has to be inside the transaction block.