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_test.go

@@ -68,6 +68,17 @@ func TestWrite_CreatesRawFile(t *testing.T) {
 	assert.Contains(t, string(content), "Some content.")
 }
 
+func TestQuery_RequiresQuery(t *testing.T) {
+	_, h := setup(t)
+	body, _ := json.Marshal(map[string]any{"limit": 5})
+	req := httptest.NewRequest(http.MethodPost, "/query", bytes.NewReader(body))
+	rec := httptest.NewRecorder()
+
+	h.Query(rec, req)
+
+	assert.Equal(t, http.StatusBadRequest, rec.Code)
+}
+
 func TestWrite_GeneratesFilenameIfAbsent(t *testing.T) {
 	dir, h := setup(t)
 	body, _ := json.Marshal(map[string]any{"content": "auto name"})