Merge remote-tracking branch 'upstream/main'

backend/migrations/087_usage_log_billing_mode.sql

+-- Add billing_mode to usage_logs (records the billing mode: token/per_request/image)
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS billing_mode VARCHAR(20);

backend/migrations/089_usage_log_image_output_tokens.sql

+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS image_output_tokens INTEGER NOT NULL DEFAULT 0;
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS image_output_cost DECIMAL(20, 10) NOT NULL DEFAULT 0;