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/types/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "@sexy.pivoine.art/types",
+  "version": "1.0.0",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": {
+      "types": "./src/index.ts"
+    }
+  },
+  "scripts": {
+    "check": "tsc --noEmit"
+  },
+  "devDependencies": {
+    "typescript": "^5.9.3"
+  }
+}