feat(db): add upstream_model column to usage_logs

Add nullable VARCHAR(100) column to record the actual model sent to upstream providers when model mapping is applied. NULL means no mapping — the requested model was used as-is.

Includes migration, concurrent index for aggregation queries, Ent schema regeneration, and migration README correction (forward-only runner, not goose).

backend/ent/schema/usage_log.go

@@ -41,6 +41,12 @@ func (UsageLog) Fields() []ent.Field {
 		field.String("model").
 			MaxLen(100).
 			NotEmpty(),
+		// UpstreamModel stores the actual upstream model name when model mapping
+		// is applied. NULL means no mapping — the requested model was used as-is.
+		field.String("upstream_model").
+			MaxLen(100).
+			Optional().
+			Nillable(),
 		field.Int64("group_id").
 			Optional().
 			Nillable(),