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/service.go

@@ -50,7 +50,7 @@ func NewService(cfg ServiceConfig) *Service {
 }
 
 // Search executes the request against local engines (in parallel) and
-// optionally upstream SearXNG for unported engines.
+// optionally the upstream instance for unported engines.
 //
 // Individual engine failures are reported as unresponsive_engines rather
 // than aborting the entire search.