feat(security): library visibility enforced at the source; per-library series; annotation ownership; admin user creation

The family-lens hardening: library_visibility was only enforced where
file bytes flowed (download/reader/OPDS) — metadata surfaces leaked
hidden libraries to any authenticated account, and registration was
open by default with no admin-side user creation.

- Series are grouped PER LIBRARY now (same name in two libraries =
  two entries: manga adaptations / LN splits no longer merge);
  /api/series aggregates only the requester's VISIBLE libraries and
  /api/series/books REQUIRES a visible library_id (breaking contract;
  web + app clients updated in the same change).
- /api/media-items, item detail, and the reader content endpoints
  (chapters/panels/outline/THUMBNAILS — page images leaked unguarded
  next to a gated GetPage) all enforce the visible set; resolveLibrary
  no longer honors a hand-typed library_id outside the visible set.
- Annotation/note/bookmark mutations (update/delete/tombstone, sync
  LWW paths incl. conflict resolution) carry ownership predicates:
  another account's bare row id is a 404, not a silent delete.
- Admin user creation: POST /api/auth/users (AdminMiddleware) + a
  Create User card in the admin Users UI — accounts stay creatable
  with self-service registration OFF, which is now the schema default
  (fresh installs; the first-admin bootstrap exception is unchanged).
- Tests: series contract re-pinned (per-library, visibility 403s),
  new ownership + admin-create integration suites, OPDS all=1 test
  now passes an explicit page size (shared-DB growth made the fresh
  books fall off page 1), device-cap helper switched to the admin
  endpoint.

templates/series.templ

@@ -76,7 +76,7 @@ templ Series(user User, seriesList []SeriesCardData, libData []LibraryData, curr
 }
 
 templ SeriesCard(series SeriesCardData) {
-	<a href={ "/series/detail?name=" + url.QueryEscape(series.Name) } class="block">
+	<a href={ "/series/detail?name=" + url.QueryEscape(series.Name) + "&library_id=" + url.QueryEscape(series.LibraryID) } class="block">
 		<div class="series-card card rounded-2xl overflow-hidden cursor-pointer" style="box-shadow: var(--shadow-card);">
 			<div class={ "stacked-covers cover-count-" + fmt.Sprintf("%d", len(series.CoverPaths)) }>
 				for i, cover := range series.CoverPaths {