rename: kafka → samsa

Full project rename from kafka to samsa (after Gregor Samsa, who
woke one morning from uneasy dreams to find himself transformed).

- Module: github.com/metamorphosis-dev/kafka → samsa
- Binary: cmd/kafka/ → cmd/samsa/
- CSS: kafka.css → samsa.css
- UI: all 'kafka' product names, titles, localStorage keys → samsa
- localStorage keys: kafka-theme → samsa-theme, kafka-engines → samsa-engines
- OpenSearch: ShortName, LongName, description, URLs updated
- AGPL headers: 'kafka' → 'samsa'
- Docs, configs, examples updated
- Cache key prefix: kafka: → samsa:

cmd/samsa/main.go

@@ -1,4 +1,4 @@
-// kafka — a privacy-respecting metasearch engine
+// samsa — a privacy-respecting metasearch engine
 // Copyright (C) 2026-present metamorphosis-dev
 //
 // This program is free software: you can redistribute it and/or modify
@@ -25,13 +25,13 @@ import (
 	"net/http"
 	"os"
 
-	"github.com/metamorphosis-dev/kafka/internal/autocomplete"
-	"github.com/metamorphosis-dev/kafka/internal/cache"
-	"github.com/metamorphosis-dev/kafka/internal/config"
-	"github.com/metamorphosis-dev/kafka/internal/httpapi"
-	"github.com/metamorphosis-dev/kafka/internal/middleware"
-	"github.com/metamorphosis-dev/kafka/internal/search"
-	"github.com/metamorphosis-dev/kafka/internal/views"
+	"github.com/metamorphosis-dev/samsa/internal/autocomplete"
+	"github.com/metamorphosis-dev/samsa/internal/cache"
+	"github.com/metamorphosis-dev/samsa/internal/config"
+	"github.com/metamorphosis-dev/samsa/internal/httpapi"
+	"github.com/metamorphosis-dev/samsa/internal/middleware"
+	"github.com/metamorphosis-dev/samsa/internal/search"
+	"github.com/metamorphosis-dev/samsa/internal/views"
 )
 
 func main() {
@@ -127,7 +127,7 @@ func main() {
 	}, logger)(handler)
 
 	addr := fmt.Sprintf(":%d", cfg.Server.Port)
-	logger.Info("kafka starting",
+	logger.Info("samsa starting",
 		"addr", addr,
 		"cache", searchCache.Enabled(),
 		"rate_limit", cfg.RateLimit.Requests > 0,