refactor: split books.rs into books+series, reorganize OpenAPI tags and fix access control

- Extract series code from books.rs into dedicated series.rs module
- Reorganize OpenAPI tags: split overloaded "books" tag into books, series, search, stats
- Add missing endpoints to OpenAPI: metadata_batch, metadata_refresh, komga, update_metadata_provider
- Add missing schemas: MissingVolumeInput, Komga/Batch/Refresh DTOs
- Fix access control: move GET /libraries and POST /libraries/:id/scan to read routes
  so non-admin tokens can list libraries and trigger scans

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

apps/api/src/search.rs

@@ -43,7 +43,7 @@ pub struct SearchResponse {
 #[utoipa::path(
     get,
     path = "/search",
-    tag = "books",
+    tag = "search",
     params(
         ("q" = String, Query, description = "Search query (books + series via PostgreSQL full-text)"),
         ("library_id" = Option<String>, Query, description = "Filter by library ID"),