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/models/+page.svelte

@@ -18,9 +18,9 @@
       .filter((model) => {
         const matchesSearch =
           searchQuery === "" ||
-          model.artist_name.toLowerCase().includes(searchQuery.toLowerCase()) ||
-          model.tags.some((tag) => tag.toLowerCase().includes(searchQuery.toLowerCase()));
-        const matchesCategory = categoryFilter === "all" || model.category === categoryFilter;
+          model.artist_name?.toLowerCase().includes(searchQuery.toLowerCase()) ||
+          model.tags?.some((tag) => tag.toLowerCase().includes(searchQuery.toLowerCase()));
+        const matchesCategory = categoryFilter === "all";
         return matchesSearch && matchesCategory;
       })
       .sort((a, b) => {
@@ -31,7 +31,7 @@
         // }
         // if (sortBy === "rating") return b.rating - a.rating;
         // if (sortBy === "videos") return b.videos - a.videos;
-        return a.artist_name.localeCompare(b.artist_name);
+        return (a.artist_name ?? "").localeCompare(b.artist_name ?? "");
       });
   });
 </script>
@@ -205,7 +205,7 @@
             <!-- Stats -->
             <div class="flex items-center justify-between text-sm text-muted-foreground mb-4">
               <!-- <span>{model.videos} videos</span> -->
-              <span class="capitalize">{model.category}</span>
+              <!-- category not available -->
             </div>
 
             <!-- Action Buttons -->