feat(db): add requested model usage log migrations

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/migrations/077_add_usage_log_requested_model.sql

@@ -0,0 +1,3 @@
+-- Add requested_model field to usage_logs for normalized request/upstream model tracking.
+-- NULL means historical rows written before requested_model dual-write was introduced.
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS requested_model VARCHAR(100);