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/tests/test_flow_proposal_l1_source.py

@@ -0,0 +1,16 @@
+import uuid
+from app.models.flow_proposal import FlowProposal
+
+
+def test_flow_proposal_accepts_l1_session_id_without_source_session():
+    p = FlowProposal(
+        account_id=uuid.uuid4(),
+        l1_session_id=uuid.uuid4(),
+        source_session_id=None,
+        proposal_type="new_flow",
+        title="AI L1 draft",
+        proposed_flow_data={"tree_structure": {"id": "root"}},
+        source="ai_realtime_l1",
+        status="pending",
+    )
+    assert p.l1_session_id is not None and p.source_session_id is None