Decide how organization merge should handle billing (Pay gem) links

🤖 From Claude:

Split out from the organization deduper (#2374).

The deduper reassigns every business association from a deleted org to the kept one, reassigns analytics/audit (ahoy_events, versions), and surfaces attached files that are deleted with the record. Billing links are deliberately left out for now because moving money-related records between orgs has real side effects that need a product decision.

Affected polymorphic tables (Pay gem): pay_customers, pay_merchants, pay_subscriptions, pay_charges, pay_payment_methods, pay_webhooks (owner/polymorphic → could be an Organization).

Current behavior: these tables are in ModelDeduper::DEFERRED_REFERENCE_TABLES, so they neither block the merge nor get moved — if a merged org had billing records, they'd stay pointing at the deleted org's id.

Decide:

• Should billing records be reassigned to the kept org on merge, left as-is, or should the merge be blocked when they exist?
• In practice, are Pay records ever owner_type = "Organization" in this app, or only Users/People? (If never orgs, we can drop the deferral entirely.)

Once decided, update ModelDeduper (reassign / block / ignore) and remove the tables from DEFERRED_REFERENCE_TABLES as appropriate.

Start with the organization deduper from #2374 and inspect ModelDeduper, especially DEFERRED_REFERENCE_TABLES and its handling of polymorphic Pay tables. Determine whether any listed records use owner_type="Organization" and establish whether merges should reassign, block, or ignore them. Done means the product decision is reflected in ModelDeduper and the deferred-table list.