PERF: Respect guardian when calculating topics per page in categories (#37259)

**What is the problem?**

`CategoriesController.topics_per_page` counted all top-level categories
regardless of user permissions. On sites with many restricted
categories, this caused excessive topic fetching for users who could
only see a small number of categories.

**What is the solution?**

Use `Category.secured(guardian)` to only count categories visible to the
current user and add a maximum cap of 100 topics as a safety net.

spec/requests/categories_controller_spec.rb

@@ -1317,6 +1317,34 @@ RSpec.describe CategoriesController do
         expect(parsed_topic).to be_present
       end
     end
+
+    it "only counts categories the user can see when calculating topics per page" do
+      SiteSetting.categories_topics = 0
+      restricted_group = Fabricate(:group)
+      4.times { Fabricate(:private_category, group: restricted_group) }
+
+      # anon sees 2 categories (uncategorized + category): 2 * 1.5 = 3, clamped to min 5
+      get "/categories_and_latest.json"
+      expect(response.parsed_body["topic_list"]["topics"].size).to eq(5)
+
+      # member sees 6 categories (2 + 4 private): 6 * 1.5 = 9
+      restricted_group.add(user)
+      sign_in(user)
+      get "/categories_and_latest.json"
+      expect(response.parsed_body["topic_list"]["topics"].size).to eq(9)
+    end
+
+    it "enforces maximum cap on topics per page" do
+      SiteSetting.categories_topics = 0
+      5.times { Fabricate(:category) }
+
+      # 7 categories (uncategorized + category + 5 new): 7 * 1.5 = 10, capped to max 7
+      stub_const(CategoriesController, :MAX_CATEGORIES_TOPICS, 7) do
+        sign_in(admin)
+        get "/categories_and_latest.json"
+        expect(response.parsed_body["topic_list"]["topics"].size).to eq(7)
+      end
+    end
   end
 
   describe "#visible_groups" do