refactor: remove SearXNG references and rename binary to kafka

- Rename cmd/searxng-go to cmd/kafka
- Remove all SearXNG references from source comments while keeping
  "SearXNG-compatible API" in user-facing docs
- Update binary paths in README, CLAUDE.md, and Dockerfile
- Update log message to "kafka starting"

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

internal/search/request_params.go

@@ -11,7 +11,7 @@ import (
 var languageCodeRe = regexp.MustCompile(`^[a-z]{2,3}(-[a-zA-Z]{2})?$`)
 
 func ParseSearchRequest(r *http.Request) (SearchRequest, error) {
-	// SearXNG supports both GET and POST and relies on form values for routing.
+	// Supports both GET and POST and relies on form values for routing.
 	if err := r.ParseForm(); err != nil {
 		return SearchRequest{}, errors.New("invalid request: cannot parse form")
 	}
@@ -90,7 +90,7 @@ func ParseSearchRequest(r *http.Request) (SearchRequest, error) {
 	// engines is an explicit list of engine names.
 	engines := splitCSV(strings.TrimSpace(r.FormValue("engines")))
 
-	// categories and category_<name> params mirror SearXNG's webadapter parsing.
+	// categories and category_<name> params mirror the webadapter parsing.
 	// We don't validate against a registry here; we just preserve the requested values.
 	catSet := map[string]bool{}
 	if catsParam := strings.TrimSpace(r.FormValue("categories")); catsParam != "" {