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/engines/engine.go

@@ -6,7 +6,7 @@ import (
 	"github.com/metamorphosis-dev/kafka/internal/contracts"
 )
 
-// Engine is a Go-native implementation of a SearXNG engine.
+// Engine is a Go-native implementation of a search engine.
 //
 // Implementations should return a SearchResponse containing only the results
 // for that engine subset; the caller will merge multiple engine responses.