feat: add batch_id/target_label to sessions and batch launch endpoint

- Add batch_id (UUID, nullable, indexed) and target_label (String 255,
  nullable) columns to the Session model
- Manual Alembic migration 6e8128ef2aa8 applies both columns
- POST /sessions/batch creates one session per target for maintenance
  flows; rejects empty targets (422) and non-maintenance trees (400)
- SessionResponse schema exposes batch_id and target_label
- 3 new integration tests, all 540 tests pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/alembic/versions/6e8128ef2aa8_add_batch_id_and_target_label_to_.py

@@ -0,0 +1,28 @@
+"""add batch_id and target_label to sessions
+
+Revision ID: 6e8128ef2aa8
+Revises: 5812e7df852f
+Create Date: 2026-02-17
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+# revision identifiers, used by Alembic.
+revision = '6e8128ef2aa8'
+down_revision = '5812e7df852f'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column('sessions', sa.Column('batch_id', postgresql.UUID(as_uuid=True), nullable=True))
+    op.add_column('sessions', sa.Column('target_label', sa.String(255), nullable=True))
+    op.create_index('ix_sessions_batch_id', 'sessions', ['batch_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_sessions_batch_id', table_name='sessions')
+    op.drop_column('sessions', 'target_label')
+    op.drop_column('sessions', 'batch_id')