chore(inventory): declare Postbox client transform endpoint

tools/inventory/endpoint-surface-declarations.json

@@ -1377,6 +1377,13 @@
       "rationale": "Postbox delivery, access-decision, and routing-preview APIs are consumed by Campaign and other module capabilities.",
       "repository": "govoplan-postbox"
     },
+    {
+      "category": "public_integration",
+      "method": "POST",
+      "path": "/postbox/admin/postboxes/{}/protection-transitions/{}/transform",
+      "rationale": "An approved external encryption client uses this governed endpoint to submit per-message E2EE or plaintext transforms with digest and authority evidence; the ordinary server-rendered WebUI never handles E2EE private keys.",
+      "repository": "govoplan-postbox"
+    },
     {
       "category": "intentionally_headless",
       "method": "POST",