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

internal/cache/cache.go

@@ -81,7 +81,7 @@ func (c *Cache) Get(ctx context.Context, key string) (contracts.SearchResponse,
 		return contracts.SearchResponse{}, false
 	}
 
-	fullKey := "gosearch:" + key
+	fullKey := "kafka:" + key
 
 	data, err := c.client.Get(ctx, fullKey).Bytes()
 	if err != nil {
@@ -113,7 +113,7 @@ func (c *Cache) Set(ctx context.Context, key string, resp contracts.SearchRespon
 		return
 	}
 
-	fullKey := "gosearch:" + key
+	fullKey := "kafka:" + key
 	if err := c.client.Set(ctx, fullKey, data, c.ttl).Err(); err != nil {
 		c.logger.Warn("cache set failed", "key", fullKey, "error", err)
 	}
@@ -124,7 +124,7 @@ func (c *Cache) Invalidate(ctx context.Context, key string) {
 	if !c.Enabled() {
 		return
 	}
-	fullKey := "gosearch:" + key
+	fullKey := "kafka:" + key
 	c.client.Del(ctx, fullKey)
 }