style(api): align struct field tags and fix string concatenation

- Fix JSON field alignment in error response and TrafficBucket struct
- Add documentation comment and swagger annotations for Breakdown field
- Remove unnecessary string concatenation spacing in SQL select

internal/api/log_handler.go

@@ -620,7 +620,8 @@ type TrafficMetrics struct {
 type TrafficBucket struct {
 	Time      string `json:"time"`
 	Timestamp int64  `json:"timestamp"`
-	Breakdown map[string]TrafficMetrics `json:"breakdown"`
+	// Breakdown contains per-model metrics, keyed by model name (e.g. "gpt-4", "claude-3-opus", "other")
+	Breakdown map[string]TrafficMetrics `json:"breakdown" swaggertype:"object,object" example:"{\"gpt-4\":{\"count\":10,\"tokens_in\":1000,\"tokens_out\":500},\"claude-3-opus\":{\"count\":5,\"tokens_in\":800,\"tokens_out\":400},\"other\":{\"count\":3,\"tokens_in\":200,\"tokens_out\":100}}"`
 	Total     TrafficMetrics            `json:"total"`
 }