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/src/graphql/resolvers/comments.ts

@@ -25,6 +25,7 @@ builder.queryField("commentsForVideo", (t) =>
               id: users.id,
               first_name: users.first_name,
               last_name: users.last_name,
+              artist_name: users.artist_name,
               avatar: users.avatar,
             })
             .from(users)
@@ -66,6 +67,7 @@ builder.mutationField("createCommentForVideo", (t) =>
           id: users.id,
           first_name: users.first_name,
           last_name: users.last_name,
+          artist_name: users.artist_name,
           avatar: users.avatar,
         })
         .from(users)