Keep max-age within an expiring image URL's lifetime (closes #63)

Both image routes sent Cache-Control: public, max-age=31536000,
immutable unconditionally, so a browser or proxy could keep serving an
image for a year after its signed or encrypted URL had expired. max-age
is now the whole seconds left until the URL expires, never negative and
at most one year; a URL with no expiry keeps one year. The 304 answer
uses the same value. An encrypted URL's expiry now reaches
ImageRequest.Expires through ToImageRequest. immutable stays: freshness
now ends no later than the URL's expiry. README.md documents the header.

Model: opus-5-5

internal/imgcache/imgcache.go

@@ -95,7 +95,8 @@ type ImageRequest struct {
 	FitMode FitMode
 	// Signature is the HMAC signature for non-allowlisted hosts
 	Signature string
-	// Expires is the signature expiration timestamp
+	// Expires is when the URL expires: the exp of a signed URL, or the expiry
+	// of an encrypted URL; the zero time if it has none
 	Expires time.Time
 	// AllowHTTP indicates whether HTTP (non-TLS) is allowed for this request
 	AllowHTTP bool