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

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+-- 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);

backend/migrations/078_add_usage_log_requested_model_index_notx.sql

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+-- Support requested_model / upstream_model aggregations with time-range filters.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_usage_logs_created_requested_model_upstream_model
+ON usage_logs (created_at, requested_model, upstream_model);