feat(l1): extend FlowProposal with source/linked_ticket/validated_by_outcome

Adds source (NOT NULL, backfilled to 'manual_draft'), linked_ticket_id,
linked_ticket_kind, validated_by_outcome columns. CHECK constraints on
source values and linked_ticket_kind values. walked_path lives on the
new l1_walk_sessions table (Task 6) — NOT on FlowProposal.

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

backend/alembic/versions/ff6fe5895ea2_extend_flow_proposals_l1.py

@@ -0,0 +1,52 @@
+"""extend_flow_proposals_l1
+
+Revision ID: ff6fe5895ea2
+Revises: a8186f22506d
+Create Date: 2026-05-28 16:26:06.932886
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'ff6fe5895ea2'
+down_revision: Union[str, None] = 'a8186f22506d'
+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('source', sa.String(30), nullable=True))
+    op.add_column('flow_proposals', sa.Column('linked_ticket_id', sa.String(64), nullable=True))
+    op.add_column('flow_proposals', sa.Column('linked_ticket_kind', sa.String(10), nullable=True))
+    op.add_column(
+        'flow_proposals',
+        sa.Column('validated_by_outcome', sa.Boolean(), nullable=False, server_default='false'),
+    )
+
+    # Backfill existing rows then enforce NOT NULL on source
+    op.execute("UPDATE flow_proposals SET source = 'manual_draft' WHERE source IS NULL")
+    op.alter_column('flow_proposals', 'source', nullable=False)
+
+    op.create_check_constraint(
+        'ck_flow_proposals_source',
+        'flow_proposals',
+        "source IN ('ai_realtime_l1', 'kb_accelerator', 'manual_draft', 'ai_promoted')",
+    )
+    op.create_check_constraint(
+        'ck_flow_proposals_linked_ticket_kind',
+        'flow_proposals',
+        "linked_ticket_kind IS NULL OR linked_ticket_kind IN ('psa', 'internal')",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint('ck_flow_proposals_linked_ticket_kind', 'flow_proposals', type_='check')
+    op.drop_constraint('ck_flow_proposals_source', 'flow_proposals', type_='check')
+    op.drop_column('flow_proposals', 'validated_by_outcome')
+    op.drop_column('flow_proposals', 'linked_ticket_kind')
+    op.drop_column('flow_proposals', 'linked_ticket_id')
+    op.drop_column('flow_proposals', 'source')