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.

database/schema/schema.sql

@@ -68,7 +68,7 @@ INSERT INTO system_settings (setting_key, setting_value, description, setting_ty
 ('password_require_lower', 'true', 'Require at least one lowercase letter (a-z)', 'bool', NULL, NULL, FALSE, 'security'),
 ('password_require_number', 'true', 'Require at least one number (0-9)', 'bool', NULL, NULL, FALSE, 'security'),
 ('password_require_special', 'true', 'Require at least one special character', 'bool', NULL, NULL, FALSE, 'security'),
-('registration_enabled', 'true', 'Allow users to create their own accounts (admins can always create accounts)', 'bool', NULL, NULL, FALSE, 'security'),
+('registration_enabled', 'false', 'Allow users to create their own accounts (admins can always create accounts)', 'bool', NULL, NULL, FALSE, 'security'),
 -- security / auth (restart required)
 ('auth_rate_limit_per_min', '10', 'Global auth API rate limit (requests per minute)', 'int', '1', '10000', TRUE, 'security'),
 ('login_max_attempts', '5', 'Failed login attempts before lockout', 'int', '1', '100', TRUE, 'security'),