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/backend/package.json

@@ -14,6 +14,7 @@
     "check": "tsc --noEmit"
   },
   "dependencies": {
+    "@sexy.pivoine.art/types": "workspace:*",
     "@fastify/cookie": "^11.0.2",
     "@fastify/cors": "^10.0.2",
     "@fastify/multipart": "^9.0.3",