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/magazine/[slug]/+page.svelte

@@ -139,6 +139,7 @@
         </div>
 
         <!-- Author Bio -->
+        {#if data.article.author}
         <Card class="p-0 bg-gradient-to-r from-card/50 to-card">
           <CardContent class="p-6">
             <div class="flex items-start gap-4">
@@ -164,15 +165,13 @@
                     >
                       {data.article.author.website}
                     </a>
-                    <!-- <a href="https://{data.article.author.social.website}" class="text-primary hover:underline">
-                    {data.article.author.social.website}
-                  </a> -->
                   </div>
                 {/if}
               </div>
             </div>
           </CardContent>
         </Card>
+        {/if}
       </article>
 
       <!-- Sidebar -->