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>

app/models/attachment.rb

@@ -166,7 +166,10 @@ class Attachment < ApplicationRecord
   end
 
   def validate_file_size(byte_size)
-    errors.add(:file, 'size is too big') if byte_size > 40.megabytes
+    limit_mb = GlobalConfigService.load('MAXIMUM_FILE_UPLOAD_SIZE', 40).to_i
+    limit_mb = 40 if limit_mb <= 0
+
+    errors.add(:file, 'size is too big') if byte_size > limit_mb.megabytes
   end
 
   def media_file?(file_content_type)