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/planner.go

@@ -48,7 +48,7 @@ func NewPlanner(portedEngines []string) *Planner {
 
 // Plan returns:
 // - localEngines: engines that are configured as ported for this service
-// - upstreamEngines: engines that should be executed by upstream SearXNG
+// - upstreamEngines: engines that should be executed by the upstream instance
 // - requestedEngines: the (possibly inferred) requested engines list
 //
 // If the request provides an explicit `engines` parameter, we use it.
@@ -80,7 +80,7 @@ func (p *Planner) Plan(req contracts.SearchRequest) (localEngines, upstreamEngin
 
 func inferFromCategories(categories []string) []string {
 	// Minimal mapping for the initial porting subset.
-	// This mirrors the idea of selecting from SearXNG categories without
+	// This mirrors the idea of selecting from engine categories without
 	// embedding the whole engine registry.
 	set := map[string]bool{}
 	for _, c := range categories {