[Subevent] Allow for downloading subevents as CSV (#11756)

## Summary of the problem

One of our partners wants to use the v3 API to fetch information
regarding each of their subevents. Right now, there's no way for them to
easily retrieve a list of subevents with their IDs.

## Describe your changes

This PR adds a CSV export to the org's Subevents page. The CSV export
includes ID, Name, Slug, and Balance.

<img width="957" height="190" alt="image"
src="https://github.com/user-attachments/assets/d42225fd-74c3-408e-8418-2a9620862a1e"
/>

Example:
<img width="514" height="100" alt="image"
src="https://github.com/user-attachments/assets/0e4c46b7-7b78-4b94-b0f9-5967fdc2a562"
/>
[The Orpheus Foundation's
sub-organizations.csv](https://github.com/user-attachments/files/22566814/The.Orpheus.Foundation.s.sub-organizations.csv)



We should maybe also consider adding a subevents route to the v3 API,
but in the meantime, this will do.

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: garyhtou

app/controllers/events_controller.rb

@@ -819,13 +819,34 @@ def employees
   def sub_organizations
     authorize @event
 
-    search = params[:q] || params[:search]
+    respond_to do |format|
+      format.html do
+        search = params[:q] || params[:search]
+
+        relation = @event.subevents
+        relation = relation.where("name ILIKE ?", "%#{search}%") if search.present?
+        relation = relation.order(created_at: :desc)
+
+        @sub_organizations = relation
+      end
 
-    relation = @event.subevents
-    relation = relation.where("name ILIKE ?", "%#{search}%") if search.present?
-    relation = relation.order(created_at: :desc)
+      # CSV export intentionally does not consider filters
+      format.csv do
+        csv = CSV.generate(headers: true) do |csv|
+          # We include the public ID because our partners iterate this CSV to
+          # access organizations via the V3 API. The public ID serves has a
+          # robust, immutable identifier compared to slugs.
+          csv << %w[ID Name Slug Balance]
+
+          @event.subevents.find_each do |e|
+            csv << [e.public_id, e.name, e.slug, e.balance_v2_cents / 100.0]
+          end
+        end
+
+        send_data csv, filename: "#{@event.name}'s sub-organizations.csv", type: "text/csv", disposition: :attachment
+      end
+    end
 
-    @sub_organizations = relation
   end
 
   def create_sub_organization

app/views/events/sub_organizations.html.erb

@@ -4,6 +4,9 @@
 
 <h1 class="heading flex">
   <span class="flex-grow">Sub-organizations</span>
+  <%= pop_icon_to "download",
+                  { format: :csv },
+                  class: "align-middle tooltipped tooltipped--w", "aria-label": "Download all as CSV" %>
   <%= link_to "#", class: "btn bg-success", data: { behavior: "modal_trigger", modal: "create" }, disabled: !policy(@event).create_sub_organization? do %>
     <%= inline_icon "plus" %>
     Create a subsidiary