case 4653 - deleting a record still in use is refused with "in use by" instead of a server error

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

devdocs/modernization/NET10-PORT.md

@@ -285,6 +285,7 @@ Found during the port, out of scope. One line each.
 - From case 4755 (tidy-up, no behaviour change): `SetNextBackup` no longer reads `NextBackup`'s previous value, so the startup line `NextBackup = FileUtil.MostRecentAutomatedBackupFileDate()` (`util/ServerGlobalOpsSettingsCache.cs:50`), the reset to `DateTime.MinValue` in `GlobalOpsBackupSettingsBiz` and `FileUtil.MostRecentAutomatedBackupFileDate` itself are dead. Same in Sockeye
 - Found in case 4755's integration run (2026-10-31): a 500 response produced by the exception handler (e.g. `/` on a debug server whose `wwwroot` has no client) has none of the three safe headers from case 4727 (F35), because the handler clears headers set before it. Integration test `ApiRoute.SafeHeadersOnEveryResponse` fails for that reason on a debug server without the client in `wwwroot`; it passes on a release build
 - Pre-existing (found in the 4632 IIS test, John: Deferred): emailed report attachments have the content type `application/pdf/pdf`, because `Mailer.cs:51` builds `new MimePart("application/pdf", "pdf")` (media type and subtype) instead of `new MimePart("application", "pdf")`. Mail programs still open the file by its `.pdf` name
+- Found in case 4653 (pre-existing): `CustomerBiz.ValidateCanDeleteAsync` refuses to delete a customer that has contacts (users), yet `DeleteAsync` then has code to delete the customer's contacts, which can never run. Its comment says contacts were meant to be deleted with the customer, and that the check is there "for now". Decide which behaviour is wanted; same in Sockeye
 
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