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.

cmd/server/tests/opds_test.go

@@ -505,7 +505,11 @@ func TestOPDSLibraryFolders(t *testing.T) {
 	})
 
 	t.Run("all=1 keeps the flat cross-library catalog", func(t *testing.T) {
-		status, contentType, body := getFeed(t, base+"/catalog?all=1")
+		// Explicit large page: the shared test DB accumulates items, and
+		// the freshly created books sort last — without a generous
+		// per_page they fall onto a later page and the assertion reads
+		// tea leaves instead of the contract.
+		status, contentType, body := getFeed(t, base+"/catalog?all=1&per_page=200")
 		require.Equal(t, http.StatusOK, status)
 		assert.Contains(t, contentType, "kind=acquisition")
 		assert.Contains(t, body, "OPDS Folder Book 1")