exam: persist exam_response_areas.meta on canvas replace-save (WS-2 item 4)

The full-replace save dropped the rich region meta (figure name/description, OMR
geometry). Add meta to ResponseAreaPayload + the replace insert so a named context
figure survives a round-trip. Pairs with the app carrying name/description through.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GruxHXxfdp4kZCgAMVFgvV

routers/exam/schemas.py

@@ -80,6 +80,9 @@ class ResponseAreaPayload(BaseModel):
     ] = None
     # Optional Context differentiation (v1 generic; future graph/chart/data_table/diagram/code_block/passage).
     context_type: Optional[str] = None
+    # Rich recognition payload (75-exam-marker-region-meta.sql): figure name/description, OMR geometry, unit…
+    # Carried on canvas save so a named context figure survives a round-trip.
+    meta: Optional[Dict[str, Any]] = None
     source: Literal["manual", "ai"] = "manual"
     confirmed: bool = True
     confidence: Optional[float] = Field(default=None, ge=0, le=1)

routers/exam/templates.py

@@ -1143,6 +1143,7 @@ async def replace_template(
                 "kind": ra.kind,
                 "response_form": ra.response_form,
                 "context_type": ra.context_type,   # 73: optional Context differentiation
+                "meta": ra.meta,                   # 75: rich recognition payload (name/description/OMR/…)
                 "source": ra.source,
                 "confirmed": ra.confirmed,
                 "confidence": ra.confidence,