fix(api): resolve all OpenAPI schema reference errors

- Add #[schema(value_type = Option<String>)] on chrono::DateTime fields
- Register SeriesPage in openapi.rs components
- Fix module-prefixed ref (index_jobs::IndexJobResponse -> IndexJobResponse)
- Strengthen test: assert all $ref targets exist in components/schemas

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/libraries.rs

@@ -18,6 +18,7 @@ pub struct LibraryResponse {
     pub book_count: i64,
     pub monitor_enabled: bool,
     pub scan_mode: String,
+    #[schema(value_type = Option<String>)]
     pub next_scan_at: Option<chrono::DateTime<chrono::Utc>>,
     pub watcher_enabled: bool,
 }