feat(exam): persist S4-9 region kinds + Part geometry; keep metadata out of graph

Backend follow-on to migration 73:
- schemas: ResponseAreaPayload.kind extended to response|context|question_number|
  mark_area|reference|furniture + context_type; QuestionPayload gains bounds+page.
- PUT serialization persists Part bounds/page and region context_type.
- Neo4j projection only emits Region nodes for response/context regions; the
  metadata kinds (question_number/mark_area/reference/furniture) are physical-layer
  only and stay out of cc.public.exams.
- Unit test: new kinds + Part geometry + context_type round-trip.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

routers/exam/templates.py

@@ -204,6 +204,8 @@ async def replace_template(
                 "mark_scheme": q.mark_scheme,
                 "is_container": q.is_container,
                 "spec_ref": q.spec_ref,
+                "bounds": q.bounds,   # drawn Part box (73); null for derived main questions
+                "page": q.page,
             }
             if q.id:
                 r["id"] = q.id
@@ -220,6 +222,7 @@ async def replace_template(
                 "bounds": ra.bounds,
                 "kind": ra.kind,
                 "response_form": ra.response_form,
+                "context_type": ra.context_type,   # 73: optional Context differentiation
                 "source": ra.source,
                 "confirmed": ra.confirmed,
                 "confidence": ra.confidence,