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/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