fix(api): clamp negative offset instead of erroring + clarify duration_sec=0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/convert.go

@@ -124,8 +124,8 @@ func trackRefFrom(t dbq.Track, albumTitle, artistName string) TrackRef {
 }
 
 // parsePaging reads limit/offset from the query string, applying defaults
-// and clamping. Returns a 400-worthy error when the values are non-numeric
-// or negative; out-of-range values silently clamp (deliberate — UX).
+// and clamping. Returns a 400-worthy error when the values are non-numeric;
+// out-of-range values silently clamp (deliberate — UX).
 func parsePaging(raw url.Values) (limit, offset int, err error) {
 	const (
 		defLimit = 50
@@ -151,7 +151,7 @@ func parsePaging(raw url.Values) (limit, offset int, err error) {
 			return 0, 0, errBadPaging
 		}
 		if n < 0 {
-			return 0, 0, errBadPaging
+			n = 0
 		}
 		offset = n
 	}