fix(ingestion): validate required query field in Query handler

Empty or whitespace-only queries would silently pass through to search,
returning meaningless results. Also removed the Domain field from
queryRequest — it was accepted but silently ignored since search.Query
has no domain parameter, which would confuse callers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/api/handler.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
@@ -25,9 +26,8 @@ func NewHandler(brainDir string, logger *slog.Logger) *Handler {
 }
 
 type queryRequest struct {
-	Query  string `json:"query"`
-	Domain string `json:"domain,omitempty"`
-	Limit  int    `json:"limit,omitempty"`
+	Query string `json:"query"`
+	Limit int    `json:"limit,omitempty"`
 }
 
 type writeRequest struct {
@@ -42,6 +42,10 @@ func (h *Handler) Query(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "invalid JSON", http.StatusBadRequest)
 		return
 	}
+	if strings.TrimSpace(req.Query) == "" {
+		http.Error(w, "query is required", http.StatusBadRequest)
+		return
+	}
 	if req.Limit == 0 {
 		req.Limit = 5
 	}