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/main_result.go

@@ -5,15 +5,15 @@ import (
 	"encoding/json"
 )
 
-// MainResult represents one element of SearXNG's `results` array.
+// MainResult represents one element of the `results` array.
 //
-// SearXNG returns many additional keys beyond what templates use. To keep the
+// The API returns many additional keys beyond what templates use. To keep the
 // contract stable for proxying/merging, we preserve all unknown keys in
 // `raw` and re-emit them via MarshalJSON.
 type MainResult struct {
 	raw map[string]any
 
-	// Common fields used by SearXNG templates (RSS uses: title, url, content, pubdate).
+	// Common fields used by templates (RSS uses: title, url, content, pubdate).
 	Template string `json:"template"`
 	Title    string `json:"title"`
 	Content  string `json:"content"`
@@ -28,12 +28,12 @@ type MainResult struct {
 	Positions []int    `json:"positions"`
 	Engines   []string `json:"engines"`
 
-	// These fields exist in SearXNG's MainResult base; keep them so downstream
+	// These fields exist in the MainResult base; keep them so downstream
 	// callers can generate richer output later.
 	OpenGroup  bool `json:"open_group"`
 	CloseGroup bool `json:"close_group"`
 
-	// parsed_url in SearXNG is emitted as a tuple; we preserve it as-is.
+	// parsed_url is emitted as a tuple; we preserve it as-is.
 	ParsedURL any `json:"parsed_url"`
 }