feat(event-subscriber): accept partial_update as a register write (refs #153)

The ACL upserts with PATCH, so every approval notification carries actie `partial_update`.
Accepting it makes the INGEDIEND → INGESCHREVEN transition project. `update` stays accepted
so a PUT-shaped write behaves the same; `destroy` deliberately does not — removing a
registration from the public register is its own decision, not a side effect of this one.

ADR-0030 records why the actie list is what it is.

docs/architecture/adr-0030-projection-sourced-from-the-register.md

@@ -34,6 +34,12 @@ notification points at. The projection is a cache of the register; ZGW is no lon
   as a kenmerk — so the record is read back through the ACL (`POST /register-records/read`).
   §8.1 applies to Objecten exactly as ADR-0028 established: the ACL is the only code that talks
   to it.
+- The accepted acties are `create`, `update` and `partial_update`. The last one is not
+  defensive breadth: the ACL upserts with PATCH, and DRF routes a PATCH through the notifying
+  `update()` while naming the action `partial_update` — which is what Objecten publishes. So
+  every approval arrives as `partial_update`, and accepting only `create`/`update` drops the
+  one state change this slice exists to project. `destroy` is deliberately not accepted:
+  removing a registration from the public register is its own decision.
 - The record already carries `id`, `status` and `reference`, so the row is the record. The
   zaak-shaped surface goes: `IsZaakCreated`, `IsZaakStatusSet`, `ZaakUrl`, `ZaakId`, and
   `ToEntry`'s `Resource == "status"` inference are replaced by `IsRegisterRecordWritten` +