Add view tokens for non-public gallery images and per-mode gate pages.

<img> tags cannot send Authorization headers, so non-public gallery photos
were invisible even to authorized viewers. The server now mints short-lived
HMAC view tokens (gallery-scoped, hour-bucketed) and embeds them in every
file URL for non-public galleries. Access denials return distinct 403
messages per visibility mode, and the frontend renders a matching gate page
(private, link-only, ticket-holders, login prompt) with an inline login
modal so visitors never leave the gallery page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

photo-api/internal/httpapi/photos.go

@@ -65,7 +65,7 @@ func (s *Server) uploadPhotos(w http.ResponseWriter, r *http.Request, _ auth.Use
 			writeError(w, uploadErr.status, uploadErr.msg)
 			return
 		}
-		created = append(created, s.photoToJSON(photo, "", true))
+		created = append(created, s.photoToJSON(photo, s.viewTokenFor(g), true))
 		position++
 	}