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>

pnpm-lock.yaml

@@ -53,6 +53,9 @@ importers:
       '@pothos/plugin-errors':
         specifier: ^4.2.0
         version: 4.9.0(@pothos/core@4.12.0(graphql@16.13.1))(graphql@16.13.1)
+      '@sexy.pivoine.art/types':
+        specifier: workspace:*
+        version: link:../types
       argon2:
         specifier: ^0.43.0
         version: 0.43.1
@@ -151,6 +154,9 @@ importers:
       '@sexy.pivoine.art/buttplug':
         specifier: workspace:*
         version: link:../buttplug
+      '@sexy.pivoine.art/types':
+        specifier: workspace:*
+        version: link:../types
       graphql:
         specifier: ^16.11.0
         version: 16.13.1
@@ -252,6 +258,12 @@ importers:
         specifier: 3.5.0
         version: 3.5.0(vite@7.3.1(@types/node@25.3.3)(jiti@2.6.1)(lightningcss@1.31.1)(terser@5.46.0)(tsx@4.21.0))
 
+  packages/types:
+    devDependencies:
+      typescript:
+        specifier: ^5.9.3
+        version: 5.9.3
+
 packages:
 
   '@antfu/install-pkg@1.1.0':