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

docker-compose.yml

@@ -1,16 +1,16 @@
-# Docker Compose for gosearch + Valkey
+# Docker Compose for kafka + Valkey
 #
 # Usage:
 #   cp config.example.toml config.toml   # edit as needed
 #   docker compose up -d
 
 services:
-  gosearch:
+  kafka:
     build: .
     ports:
       - "8080:8080"
     volumes:
-      - ./config.toml:/etc/gosearch/config.toml:ro
+      - ./config.toml:/etc/kafka/config.toml:ro
     depends_on:
       valkey:
         condition: service_healthy