perf: use Redis for favicon cache with 24h TTL

Favicons are now cached in Valkey/Redis instead of an in-memory map:
- TTL: 24 hours (up from 1 hour in-memory)
- ETag derived from body SHA256 (no extra storage needed)
- Falls back to in-memory on cache miss when Valkey is unavailable
- GetBytes/SetBytes added to cache package for raw byte storage

In-memory faviconCache map, sync.RWMutex, and time-based expiry
logic removed from handlers.go.

cmd/samsa/main.go

@@ -77,7 +77,7 @@ func main() {
 
 	acSvc := autocomplete.NewService(cfg.Upstream.URL, cfg.HTTPTimeout())
 
-	h := httpapi.NewHandler(svc, acSvc.Suggestions, cfg.Server.SourceURL)
+	h := httpapi.NewHandler(svc, acSvc.Suggestions, cfg.Server.SourceURL, searchCache)
 
 	mux := http.NewServeMux()