feat(core): image hosts (server, litterbox, catbox, direct) and per-image rehosting

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/hosting/server.ts

@@ -0,0 +1,8 @@
+import { viaServer, type ImageHost } from "./host";
+
+/** Our server's covers folder; swept after 30 days (spec §4.6). */
+export const serverHost: ImageHost = {
+  id: "server",
+  lifetime: () => "30 days",
+  upload: (image, ctx) => viaServer(image, ctx, { host: "store" }),
+};