feat: allow configuring attachment upload limit (#12835)

## Summary
- add a configurable MAXIMUM_FILE_UPLOAD_SIZE installation setting and
surface it through super admin and global config payloads
- apply the configurable limit to attachment validations and shared
upload helpers on dashboard and widget
- introduce a reusable helper with unit tests for parsing the limit and
extend attachment specs for configurability


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[Codex
Task](https://chatgpt.com/codex/tasks/task_e_6912644786b08326bc8dee9401af6d0a)

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Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>

spec/models/attachment_spec.rb

@@ -154,4 +154,39 @@ RSpec.describe Attachment do
       end
     end
   end
+
+  describe 'file size validation' do
+    let(:attachment) { message.attachments.new(account_id: message.account_id, file_type: :image) }
+
+    before do
+      allow(GlobalConfigService).to receive(:load).and_call_original
+    end
+
+    it 'respects configured limit' do
+      allow(GlobalConfigService).to receive(:load)
+        .with('MAXIMUM_FILE_UPLOAD_SIZE', 40)
+        .and_return('5')
+
+      attachment.errors.clear
+      attachment.send(:validate_file_size, 4.megabytes)
+
+      expect(attachment.errors[:file]).to be_empty
+
+      attachment.errors.clear
+      attachment.send(:validate_file_size, 6.megabytes)
+
+      expect(attachment.errors[:file]).to include('size is too big')
+    end
+
+    it 'falls back to default when configured limit is invalid' do
+      allow(GlobalConfigService).to receive(:load)
+        .with('MAXIMUM_FILE_UPLOAD_SIZE', 40)
+        .and_return('-10')
+
+      attachment.errors.clear
+      attachment.send(:validate_file_size, 41.megabytes)
+
+      expect(attachment.errors[:file]).to include('size is too big')
+    end
+  end
 end