feat(l1): FlowProposal l1_session_id source linkage (nullable source_session_id + exactly-one check)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/flow_proposal.py

@@ -19,7 +19,10 @@ class FlowProposalSummary(BaseModel):
     supporting_session_count: int
     status: str
     target_flow_id: UUID | None = None
-    source_session_id: UUID
+    # Exactly one source is set: source_session_id (FlowPilot ai_session) XOR
+    # l1_session_id (L1 ai_build walk). Both are nullable on the model.
+    source_session_id: UUID | None = None
+    l1_session_id: UUID | None = None
     created_at: datetime
 
     model_config = {"from_attributes": True}