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/device_cap_test.go

@@ -336,7 +336,7 @@ func createTestUserForMaxDevices(t *testing.T, ts *httptest.Server, adminToken s
 	}
 
 	jsonData, _ := json.Marshal(createUserPayload)
-	req, _ := http.NewRequest("POST", ts.URL+"/api/auth/register", bytes.NewBuffer(jsonData))
+	req, _ := http.NewRequest("POST", ts.URL+"/api/auth/users", bytes.NewBuffer(jsonData))
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Authorization", "Bearer "+adminToken)
 
@@ -416,14 +416,19 @@ func createTestUserForMaxDevices(t *testing.T, ts *httptest.Server, adminToken s
 	err = json.NewDecoder(resp.Body).Decode(&result)
 	require.NoError(t, err)
 
-	// Check if user creation was successful
-	if result["user"] == nil {
+	// Admin create returns the user object at the top level; the legacy
+	// register endpoint nested it under "user". Accept both.
+	userObj, ok := result["user"].(map[string]interface{})
+	if !ok {
+		userObj = result
+	}
+	if userObj["id"] == nil {
 		// User creation failed for another reason
 		t.Logf("User creation failed, response: %+v", result)
 		return uuid.UUID{}
 	}
 
-	userIDStr := result["user"].(map[string]interface{})["id"].(string)
+	userIDStr, _ := userObj["id"].(string)
 	userID, _ := uuid.Parse(userIDStr)
 
 	return userID