feat: add shared @sexy.pivoine.art/types package and fix type safety across frontend/backend

- Create packages/types with shared TypeScript domain model interfaces (User, Video, Model, Article, Comment, Recording, etc.)
- Wire both frontend and backend packages to use @sexy.pivoine.art/types via workspace:*
- Update backend Pothos objectRef types to use shared interfaces instead of inline types
- Update frontend $lib/types.ts to re-export from shared package
- Fix all type errors introduced by more accurate nullable types (avatar/banner as string|null UUIDs, author nullable, events/device_info as object[])
- Add artist_name to comment user select in backend resolver
- Widen utility function signatures (getAssetUrl, getUserInitials, calcReadingTime) to accept null/undefined

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

packages/frontend/src/routes/sitemap.xml/+server.ts

@@ -7,7 +7,7 @@ export const GET = async () => {
     excludeRoutePatterns: ["^/signup/verify", "^/password/reset", "^/me", "^/play", "^/tags/.+"],
     paramValues: {
       "/magazine/[slug]": (await getArticles(fetch)).map((a) => a.slug),
-      "/models/[slug]": (await getModels(fetch)).map((a) => a.slug),
+      "/models/[slug]": (await getModels(fetch)).map((a) => a.slug).filter((s): s is string => s !== null),
       "/videos/[slug]": (await getVideos(fetch)).map((a) => a.slug),
     },
     defaultChangefreq: "always",