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/contracts/types.go

@@ -1,6 +1,6 @@
 package contracts
 
-// OutputFormat matches SearXNG's `/search?format=...` values.
+// OutputFormat matches the `/search?format=...` values.
 type OutputFormat string
 
 const (
@@ -28,7 +28,7 @@ type SearchRequest struct {
 	Engines    []string
 	Categories []string
 
-	// EngineData matches SearXNG's `engine_data-<engine>-<key>=<value>` parameters.
+	// EngineData matches the `engine_data-<engine>-<key>=<value>` parameters.
 	EngineData map[string]map[string]string
 
 	// AccessToken is an optional request token used to gate paid/limited engines.
@@ -36,7 +36,7 @@ type SearchRequest struct {
 	AccessToken string
 }
 
-// SearchResponse matches the JSON schema returned by SearXNG's `webutils.get_json_response()`.
+// SearchResponse matches the JSON schema used by `webutils.get_json_response()`.
 type SearchResponse struct {
 	Query                string         `json:"query"`
 	NumberOfResults     int            `json:"number_of_results"`