fix(uploads): delete the dead POST /registrations (RB-06)

POST /registrations passed its Documents list straight to Submit, which calls
DocumentStore.Link on every digital documentId in it — and linking a document
blocks its owner from ever deleting it (DeleteOwned returns 409 Linked). That
path had no ForeignIds ownership check, so any authenticated citizen could
post another citizen's document id and permanently block them from deleting
their own diploma scan. POST /applications/{id}/submit, the endpoint actually
in use, has had that guard since it was written.

Deleted rather than guarded: the endpoint is dead. No frontend caller, and
the whole registratie flow goes through /applications/{id}/submit.
RegistratieRequest went with it, and so did SubmissionRules.RejectRegistratie
— reachable only from here, and contradicted by the live path, which treats a
handmatig diploma as "does not auto-approve" rather than a 422 rejection. Its
own message said as much while being returned as a rejection. That last part
is a judgement call beyond the ticket's wording; reverting the two
SubmissionRules hunks restores it in isolation.

Coverage moved rather than vanished: the problem+json shape assertion is now
on /change-requests (the other endpoint on the same Submit helper), and the
linked-delete 409 test goes through the real submit path.

swagger.json, the generated client and the behaviour spec regenerated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/project/refactor-backlog-setup/refactor-backlog/implementation/rb-02.md

@@ -10,7 +10,7 @@ Status: **implemented** · 2026-08-27 · Source findings: `07-bio2-compliance.md
 audit page — so a BSN was written to durable storage and shown in a UI, on the one trail
 four documents describe as data-minimised and PII-free.
 
-The endpoint is the *BIG-nummer reveal*, whose own comment says the audit carries
+The endpoint is the _BIG-nummer reveal_, whose own comment says the audit carries
 "NO PII. Never the value that was (or wasn't) revealed" — and it did not carry the
 BIG-nummer. It carried the BSN instead, in the adjacent argument.
 
@@ -28,10 +28,10 @@ value-asserting test is part of this ticket's definition of done rather than a f
 
 ## What changed
 
-| File                    | Change                                                                                             |
-| ----------------------- | ---------------------------------------------------------------------------------------------------- |
-| `Program.cs`            | resource ref is `"brief"`; a comment records why the id added nothing                              |
-| `AuthzAuditTests.cs`    | **new** `No_audit_row_carries_a_subjects_bsn` — asserts on stored **values**, every string field   |
+| File                 | Change                                                                                           |
+| -------------------- | ------------------------------------------------------------------------------------------------ |
+| `Program.cs`         | resource ref is `"brief"`; a comment records why the id added nothing                            |
+| `AuthzAuditTests.cs` | **new** `No_audit_row_carries_a_subjects_bsn` — asserts on stored **values**, every string field |
 
 No identifier was lost. `BriefStore` keys one brief per owner, so `brief/<bsn>` named the
 same thing the row's acting principal already implies; there is no second brief the ref