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>

cmd/kafka/main.go

@@ -103,7 +103,7 @@ func main() {
 	}, logger)(handler)
 
 	addr := fmt.Sprintf(":%d", cfg.Server.Port)
-	logger.Info("searxng-go starting",
+	logger.Info("kafka starting",
 		"addr", addr,
 		"cache", searchCache.Enabled(),
 		"rate_limit", cfg.RateLimit.Requests > 0,