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/schemas.py

@@ -57,17 +57,24 @@ class QuestionPayload(BaseModel):
     mark_scheme: Dict[str, Any] = Field(default_factory=dict)
     is_container: bool = False
     spec_ref: Optional[str] = None
+    # Drawn Part box geometry (73-exam-marker-regions.sql). Null for derived main questions.
+    bounds: Optional[Dict[str, Any]] = None  # {x,y,w,h}
+    page: Optional[int] = None
 
 
 class ResponseAreaPayload(BaseModel):
-    id: Optional[str] = None  # == Neo4j Region.uuid_string
+    id: Optional[str] = None  # == Neo4j Region.uuid_string (only response/context project)
     question_id: str
     page: int
     bounds: Dict[str, Any]  # {x,y,w,h}
-    kind: Literal["response", "context"]
+    # S4-9 taxonomy (73-exam-marker-regions.sql): response/context graded-or-stimulus;
+    # question_number/mark_area = physical metadata; reference = student resource; furniture = ignore.
+    kind: Literal["response", "context", "question_number", "mark_area", "reference", "furniture"]
     response_form: Optional[
         Literal["lines", "answer-box", "working", "diagram", "tick-boxes", "table", "blanks"]
     ] = None
+    # Optional Context differentiation (v1 generic; future graph/chart/data_table/diagram/code_block/passage).
+    context_type: Optional[str] = None
     source: Literal["manual", "ai"] = "manual"
     confirmed: bool = True
     confidence: Optional[float] = None