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/videos/[slug]/+page.svelte

@@ -377,7 +377,7 @@
                 <div class="flex gap-3 mb-6">
                   <Avatar class="h-8 w-8 ring-2 ring-accent/20 transition-all duration-200">
                     <AvatarImage
-                      src={getAssetUrl(data.authStatus.user!.avatar.id, "mini")}
+                      src={getAssetUrl(data.authStatus.user!.avatar, "mini")}
                       alt={data.authStatus.user!.artist_name}
                     />
                     <AvatarFallback
@@ -432,27 +432,27 @@
                 <div class="space-y-4">
                   {#each data.comments as comment (comment.id)}
                     <div class="flex gap-3">
-                      <a href="/users/{comment.user_created.id}" class="flex-shrink-0">
+                      <a href="/users/{comment.user?.id}" class="flex-shrink-0">
                         <Avatar
                           class="h-8 w-8 ring-2 ring-accent/20 hover:ring-primary/40 transition-all duration-200 cursor-pointer"
                         >
                           <AvatarImage
-                            src={getAssetUrl(comment.user_created.avatar as string, "mini")}
-                            alt={comment.user_created.artist_name}
+                            src={getAssetUrl(comment.user?.avatar, "mini")}
+                            alt={comment.user?.artist_name}
                           />
                           <AvatarFallback
                             class="bg-gradient-to-br from-primary to-accent text-primary-foreground text-xs font-semibold transition-all duration-200"
                           >
-                            {getUserInitials(comment.user_created.artist_name)}
+                            {getUserInitials(comment.user?.artist_name)}
                           </AvatarFallback>
                         </Avatar>
                       </a>
                       <div class="flex-1">
                         <div class="flex items-center gap-2 mb-1">
                           <a
-                            href="/users/{comment.user_created.id}"
+                            href="/users/{comment.user?.id}"
                             class="font-medium text-sm hover:text-primary transition-colors"
-                            >{comment.user_created.artist_name}</a
+                            >{comment.user?.artist_name}</a
                           >
                           <span class="text-xs text-muted-foreground"
                             >{timeAgo.format(new Date(comment.date_created))}</span