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/alembic/versions/1fd88a68b145_flow_proposal_l1_source_linkage.py

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+"""flow_proposal l1 source linkage
+
+Revision ID: 1fd88a68b145
+Revises: cb9e282267d2
+Create Date: 2026-05-29 19:33:09.188681
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+
+# revision identifiers, used by Alembic.
+revision: str = '1fd88a68b145'
+down_revision: Union[str, None] = 'cb9e282267d2'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "flow_proposals",
+        sa.Column("l1_session_id", postgresql.UUID(as_uuid=True), nullable=True),
+    )
+    op.create_index(
+        "ix_flow_proposals_l1_session_id",
+        "flow_proposals",
+        ["l1_session_id"],
+    )
+    op.create_foreign_key(
+        "fk_flow_proposals_l1_session_id",
+        "flow_proposals",
+        "l1_walk_sessions",
+        ["l1_session_id"],
+        ["id"],
+        ondelete="SET NULL",
+    )
+    op.alter_column("flow_proposals", "source_session_id", nullable=True)
+    op.create_check_constraint(
+        "ck_flow_proposals_exactly_one_source",
+        "flow_proposals",
+        "(source_session_id IS NOT NULL) <> (l1_session_id IS NOT NULL)",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "ck_flow_proposals_exactly_one_source",
+        "flow_proposals",
+        type_="check",
+    )
+    op.alter_column("flow_proposals", "source_session_id", nullable=False)
+    op.drop_constraint(
+        "fk_flow_proposals_l1_session_id",
+        "flow_proposals",
+        type_="foreignkey",
+    )
+    op.drop_index("ix_flow_proposals_l1_session_id", "flow_proposals")
+    op.drop_column("flow_proposals", "l1_session_id")