feat(l1): add ai_build session kind (model + migration)

Teaches l1_walk_sessions a new session_kind='ai_build' for AI-generated
decision-tree walks. FK shape matches adhoc: both flow_id and
flow_proposal_id must be NULL. Drops and recreates the two affected CHECK
constraints (session_kind allowlist + target_consistency). Migration
beca7464b6b4 chains from b3358ba0e48c.

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

backend/alembic/versions/beca7464b6b4_add_ai_build_session_kind.py

@@ -0,0 +1,48 @@
+"""add ai_build session kind
+
+Revision ID: beca7464b6b4
+Revises: b3358ba0e48c
+Create Date: 2026-05-29 18:41:38.601537
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'beca7464b6b4'
+down_revision: Union[str, None] = 'b3358ba0e48c'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_constraint("ck_l1_walk_sessions_session_kind", "l1_walk_sessions", type_="check")
+    op.create_check_constraint(
+        "ck_l1_walk_sessions_session_kind", "l1_walk_sessions",
+        "session_kind IN ('flow', 'proposal', 'adhoc', 'ai_build')",
+    )
+    op.drop_constraint("ck_l1_walk_sessions_target_consistency", "l1_walk_sessions", type_="check")
+    op.create_check_constraint(
+        "ck_l1_walk_sessions_target_consistency", "l1_walk_sessions",
+        "(session_kind = 'flow' AND flow_id IS NOT NULL AND flow_proposal_id IS NULL) "
+        "OR (session_kind = 'proposal' AND flow_proposal_id IS NOT NULL AND flow_id IS NULL) "
+        "OR (session_kind IN ('adhoc', 'ai_build') AND flow_id IS NULL AND flow_proposal_id IS NULL)",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_l1_walk_sessions_target_consistency", "l1_walk_sessions", type_="check")
+    op.create_check_constraint(
+        "ck_l1_walk_sessions_target_consistency", "l1_walk_sessions",
+        "(session_kind = 'flow' AND flow_id IS NOT NULL AND flow_proposal_id IS NULL) "
+        "OR (session_kind = 'proposal' AND flow_proposal_id IS NOT NULL AND flow_id IS NULL) "
+        "OR (session_kind = 'adhoc' AND flow_id IS NULL AND flow_proposal_id IS NULL)",
+    )
+    op.drop_constraint("ck_l1_walk_sessions_session_kind", "l1_walk_sessions", type_="check")
+    op.create_check_constraint(
+        "ck_l1_walk_sessions_session_kind", "l1_walk_sessions",
+        "session_kind IN ('flow', 'proposal', 'adhoc')",
+    )

backend/app/models/l1_walk_session.py

@@ -30,6 +30,7 @@ class L1WalkSession(Base):
     - flow: Walking a published flow (flow_id required, flow_proposal_id null).
     - proposal: Walking a draft flow proposal (flow_proposal_id required, flow_id null).
     - adhoc: Free-form investigation (both flow_id and flow_proposal_id null).
+    - ai_build: AI-generated decision-tree walk (both flow_id and flow_proposal_id null).
 
     status lifecycle:
     - active: Session is in progress.
@@ -45,7 +46,7 @@ class L1WalkSession(Base):
             name="ck_l1_walk_sessions_ticket_kind",
         ),
         CheckConstraint(
-            "session_kind IN ('flow', 'proposal', 'adhoc')",
+            "session_kind IN ('flow', 'proposal', 'adhoc', 'ai_build')",
             name="ck_l1_walk_sessions_session_kind",
         ),
         CheckConstraint(
@@ -55,7 +56,7 @@ class L1WalkSession(Base):
         CheckConstraint(
             "(session_kind = 'flow' AND flow_id IS NOT NULL AND flow_proposal_id IS NULL) "
             "OR (session_kind = 'proposal' AND flow_proposal_id IS NOT NULL AND flow_id IS NULL) "
-            "OR (session_kind = 'adhoc' AND flow_id IS NULL AND flow_proposal_id IS NULL)",
+            "OR (session_kind IN ('adhoc', 'ai_build') AND flow_id IS NULL AND flow_proposal_id IS NULL)",
             name="ck_l1_walk_sessions_target_consistency",
         ),
     )

backend/tests/test_l1_ai_build_model.py

@@ -0,0 +1,16 @@
+import uuid
+
+from app.models.l1_walk_session import L1WalkSession
+
+
+def test_ai_build_session_kind_allowed_by_model_constraint():
+    """ai_build is a valid session_kind with both target FKs null (like adhoc)."""
+    s = L1WalkSession(
+        account_id=uuid.uuid4(),
+        created_by_user_id=uuid.uuid4(),
+        ticket_id="t1",
+        ticket_kind="internal",
+        session_kind="ai_build",
+    )
+    assert s.session_kind == "ai_build"
+    assert s.flow_id is None and s.flow_proposal_id is None