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/lib/components/header/header.svelte

@@ -128,7 +128,7 @@
               user={{
                 name:
                   authStatus.user!.artist_name || authStatus.user!.email.split("@")[0] || "User",
-                avatar: getAssetUrl(authStatus.user!.avatar?.id, "mini")!,
+                avatar: getAssetUrl(authStatus.user!.avatar, "mini")!,
                 email: authStatus.user!.email,
               }}
               onLogout={handleLogout}
@@ -171,7 +171,7 @@
               <div class="relative flex items-center gap-4">
                 <Avatar class="h-14 w-14 ring-2 ring-primary/30">
                   <AvatarImage
-                    src={getAssetUrl(authStatus.user!.avatar?.id, "mini")}
+                    src={getAssetUrl(authStatus.user!.avatar, "mini")}
                     alt={authStatus.user!.artist_name}
                   />
                   <AvatarFallback