chore: rename project from gosearch to kafka

A search engine named after a man who proved answers don't exist.

Renamed everywhere user-facing:
- Brand name, UI titles, OpenSearch description, CSS filename
- Docker service name, NixOS module (services.kafka)
- Cache key prefix (kafka:), User-Agent strings (kafka/0.1)
- README, config.example.toml, flake.nix descriptions

Kept unchanged (internal):
- Go module path: github.com/ashie/gosearch
- Git repository URL: git.ashisgreat.xyz/penal-colony/gosearch
- Binary entrypoint: cmd/searxng-go

config.example.toml

@@ -1,4 +1,4 @@
-# gosearch configuration
+# kafka configuration
 # Copy to config.toml and adjust as needed.
 # Environment variables are used as fallbacks when a config field is empty/unset.
 
@@ -10,7 +10,7 @@ port = 8080
 http_timeout = "10s"
 
 # Public base URL for OpenSearch XML (env: BASE_URL)
-# Set this so browsers can add gosearch as a search engine.
+# Set this so browsers can add kafka as a search engine.
 # Example: "https://search.example.com"
 base_url = ""