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>

config.example.toml

@@ -15,13 +15,13 @@ http_timeout = "10s"
 base_url = ""
 
 [upstream]
-# URL of an upstream SearXNG instance for unported engines (env: UPSTREAM_SEARXNG_URL)
+# URL of an upstream metasearch instance for unported engines (env: UPSTREAM_SEARXNG_URL)
 # Leave empty to run without an upstream proxy.
 url = ""
 
 [engines]
 # Comma-separated list of engines to execute locally in Go (env: LOCAL_PORTED_ENGINES)
-# Engines not listed here will be proxied to upstream SearXNG.
+# Engines not listed here will be proxied to the upstream instance.
 local_ported = ["wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing"]
 
 [engines.brave]