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

Both image routes sent Cache-Control: public, max-age=31536000,
immutable, so a browser or proxy could keep serving an image for a year
after its signed or encrypted URL had expired. The header is now built
from the request's Expires: max-age is the whole seconds left until the
URL expires, never negative, or one year for a URL with no expiry.
ToImageRequest now carries an encrypted URL's expiry onto the request,
as the image route already does with exp. immutable stays: it only stops
revalidation while a copy is fresh, and freshness now ends at the
expiry. README.md documents the header.

Model: opus-5-5

internal/handlers/image.go

@@ -220,6 +220,24 @@ func (s *Handlers) respondImageError(
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 
+// cacheControl returns the Cache-Control header for an image served through a
+// URL that expires at expires, or never when expires is the zero time. A cache
+// may keep the image for a year, but not past the URL's expiry, after which
+// pixa refuses the URL. The seconds left are rounded down and never negative.
+// immutable only stops revalidation while the image is fresh, so it also ends
+// at the expiry.
+func cacheControl(expires time.Time) string {
+	const oneYear = 365 * 24 * time.Hour
+
+	maxAge := oneYear
+
+	if !expires.IsZero() {
+		maxAge = min(maxAge, max(time.Until(expires), 0))
+	}
+
+	return fmt.Sprintf("public, max-age=%d, immutable", int64(maxAge/time.Second))
+}
+
 // writeImageResponse writes headers and streams the image content,
 // handling conditional and HEAD requests.
 func (s *Handlers) writeImageResponse(
@@ -235,7 +253,7 @@ func (s *Handlers) writeImageResponse(
 	}
 
 	// Cache control headers
-	w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+	w.Header().Set("Cache-Control", cacheControl(req.Expires))
 	w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
 	if resp.ETag != "" {