feat(l1): start_ai_build_session

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

backend/app/services/l1_session_service.py

@@ -98,6 +98,28 @@ async def start_adhoc_session(
     return session
 
 
+async def start_ai_build_session(
+    db: AsyncSession,
+    *,
+    account_id: UUID,
+    user: User,
+    ticket_id: str,
+    ticket_kind: str,
+) -> L1WalkSession:
+    """Start an AI-built tree session (nodes generated on demand via next-node)."""
+    session = L1WalkSession(
+        account_id=account_id,
+        created_by_user_id=user.id,
+        acting_as=_resolve_acting_as(user),
+        ticket_id=ticket_id,
+        ticket_kind=ticket_kind,
+        session_kind="ai_build",
+    )
+    db.add(session)
+    await db.flush()
+    return session
+
+
 async def record_step(
     db: AsyncSession,
     *,