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/views/templates/index.html

@@ -6,7 +6,7 @@
       <circle cx="11" cy="11" r="8"/>
       <path d="m21 21-4.35-4.35"/>
     </svg>
-    <span class="home-logo-text">kafka</span>
+    <span class="home-logo-text">samsa</span>
   </a>
   <p class="home-tagline">Private meta-search, powered by open source.</p>