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.

internal/httpapi/httpapi_test.go

@@ -67,7 +67,7 @@ func newTestServer(t *testing.T) (*httptest.Server, *httpapi.Handler) {
 		EnginesConfig: nil,
 	})
 
-	h := httpapi.NewHandler(svc, nil, "https://src.example.com")
+	h := httpapi.NewHandler(svc, nil, "https://src.example.com", nil)
 
 	mux := http.NewServeMux()
 	mux.HandleFunc("/healthz", h.Healthz)