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]>

modules/database/services/exam_projection.py

@@ -138,7 +138,12 @@ def project_template(template_id: str) -> Dict[str, Any]:
                 counts["assesses"] += (r["n"] if r else 0)
 
         # 6. Region nodes + HAS_REGION edges.
+        # Only response/context regions are part of the knowledge graph (RegionNode.kind). The other
+        # S4-9 kinds (question_number, mark_area, reference, furniture) are physical-layer metadata
+        # about the paper, not curriculum structure — they stay in Supabase, out of cc.public.exams.
         for rg in regions:
+            if rg.get("kind") not in ("response", "context"):
+                continue
             s.run(
                 "MERGE (r:Region {uuid_string:$uid}) "
                 "SET r.exam_code=$ec, r.page=$page, r.kind=$kind, r.response_form=$rf, r.node_storage_path=$nsp",