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/upstream/client.go

@@ -68,7 +68,7 @@ func (c *Client) SearchJSON(ctx context.Context, req contracts.SearchRequest, en
 
 	for engineName, kv := range req.EngineData {
 		for key, value := range kv {
-			// Mirror SearXNG's naming: `engine_data-<engine>-<key>=<value>`
+			// Mirror the naming convention: `engine_data-<engine>-<key>=<value>`
 			form.Set(fmt.Sprintf("engine_data-%s-%s", engineName, key), value)
 		}
 	}