fix: add status_update to step_type CHECK constraint

The generate_status_update service inserted AISessionStep with
step_type='status_update' which violated the DB CHECK constraint,
causing a 500 error. Also fix incorrect field name confidence_score
(should be confidence_at_step) and remove nonexistent confidence_tier.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/069_add_status_update_step_type.py

@@ -0,0 +1,31 @@
+"""add status_update to ai_session_steps step_type constraint
+
+Revision ID: 069
+Revises: 068
+Create Date: 2026-03-29
+"""
+from alembic import op
+
+
+revision = "069"
+down_revision = "068"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.drop_constraint("ck_ai_session_steps_step_type", "ai_session_steps", type_="check")
+    op.create_check_constraint(
+        "ck_ai_session_steps_step_type", "ai_session_steps",
+        "step_type IN ('question', 'action', 'script_generation', 'verification', "
+        "'info_request', 'note', 'intake_analysis', 'fork', 'status_update')",
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_ai_session_steps_step_type", "ai_session_steps", type_="check")
+    op.create_check_constraint(
+        "ck_ai_session_steps_step_type", "ai_session_steps",
+        "step_type IN ('question', 'action', 'script_generation', 'verification', "
+        "'info_request', 'note', 'intake_analysis', 'fork')",
+    )