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:

internal/cache/cache.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
@@ -25,7 +25,7 @@ import (
 	"log/slog"
 	"time"
 
-	"github.com/metamorphosis-dev/kafka/internal/contracts"
+	"github.com/metamorphosis-dev/samsa/internal/contracts"
 	"github.com/redis/go-redis/v9"
 )
 
@@ -97,7 +97,7 @@ func (c *Cache) Get(ctx context.Context, key string) (contracts.SearchResponse,
 		return contracts.SearchResponse{}, false
 	}
 
-	fullKey := "kafka:" + key
+	fullKey := "samsa:" + key
 
 	data, err := c.client.Get(ctx, fullKey).Bytes()
 	if err != nil {
@@ -129,7 +129,7 @@ func (c *Cache) Set(ctx context.Context, key string, resp contracts.SearchRespon
 		return
 	}
 
-	fullKey := "kafka:" + key
+	fullKey := "samsa:" + key
 	if err := c.client.Set(ctx, fullKey, data, c.ttl).Err(); err != nil {
 		c.logger.Warn("cache set failed", "key", fullKey, "error", err)
 	}
@@ -140,7 +140,7 @@ func (c *Cache) Invalidate(ctx context.Context, key string) {
 	if !c.Enabled() {
 		return
 	}
-	fullKey := "kafka:" + key
+	fullKey := "samsa:" + key
 	c.client.Del(ctx, fullKey)
 }