feat(zgw): finish WP-52 OpenZaak Notificaties (NRC) webhook slice

Endpoint/DTO/options landed already in c4dd846; this closes the loop with
NotificatieTests.cs (accept/reject/missing-header, asserting the AuthzAuditStore
row), missing appsettings.json keys (also backfills DrcBaseUrl/
InformatieobjecttypeUrls, stale since WP-51), and the webhook + abonnement
provisioning docs.

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

backend/src/BigRegister.Api/appsettings.json

@@ -6,7 +6,7 @@
     }
   },
   "AllowedHosts": "*",
-  "_Zgw": "WP-49/50: set Enabled=true + the URLs/credentials/RSINs/zaaktype map to source + create cases against a real OpenZaak. Off = local SQLite store (offline POC default).",
+  "_Zgw": "WP-49..52: set Enabled=true + the URLs/credentials/RSINs/type maps to source, create and document cases against a real OpenZaak; NrcBaseUrl/NotificatieAuthorization configure the inbound notificaties webhook. Off = local SQLite store (offline POC default).",
   "Zgw": {
     "Enabled": false,
     "ZrcBaseUrl": "",
@@ -21,6 +21,10 @@
       "registratie": "",
       "herregistratie": "",
       "intake": ""
-    }
+    },
+    "DrcBaseUrl": "",
+    "InformatieobjecttypeUrls": {},
+    "NrcBaseUrl": "",
+    "NotificatieAuthorization": ""
   }
 }