kiwix, weather: name the client timeout constant (V-581)

Both clients used a bare 10*time.Second literal for the http.Client
timeout, unlike websearch.DefaultTimeout which carries a comment
explaining the number. Naming them puts the reason (LAN ZIM read vs.
a public API over the internet) next to the value; the constants
equal what was there before, so behaviour is unchanged.

internal/weather/openmeteo.go

@@ -13,13 +13,19 @@ import (
 	"github.com/kami/maven/internal/morph"
 )
 
+// clientTimeout — the whole request, geocode or forecast. Both are one call to
+// a public API over the internet rather than a LAN service, hence longer than
+// a bare "it's slow" budget; there is no retry behind it, so a slow reply
+// still costs the caller the whole wait.
+const clientTimeout = 10 * time.Second
+
 type OpenMeteoProvider struct {
 	httpClient *http.Client
 }
 
 func NewOpenMeteoProvider() *OpenMeteoProvider {
 	return &OpenMeteoProvider{
-		httpClient: &http.Client{Timeout: 10 * time.Second},
+		httpClient: &http.Client{Timeout: clientTimeout},
 	}
 }