fix(email): allow larger email templates (#15146)

## Description

Branded email layouts and other email templates can now contain up to
262,144 characters instead of the generic 20,000-character text limit.
This supports customer layouts around 41 KB and 100 KB while retaining a
defined application-level ceiling. The account and inbox API schemas now
expose the same maximum.

## Closes


[CW-7682](https://linear.app/chatwoot/issue/CW-7682/allow-larger-branded-email-templates)

Related:
[CW-7514](https://linear.app/chatwoot/issue/CW-7514/branded-html-email-templates-per-inboxbrand)

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] This change requires a documentation update

## How Has This Been Tested?

1. As an administrator with `branded_email_templates` enabled, update an
account branded layout with a 100 KB Liquid layout containing `{{
content_for_layout }}` and confirm the request succeeds.
2. Update an Email inbox layout with more than 262,144 characters and
confirm the API returns `422`.
3. Confirm a layout at exactly 262,144 characters remains valid.

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

app/models/email_template.rb

@@ -22,6 +22,7 @@
 class EmailTemplate < ApplicationRecord
   BRANDED_LAYOUT_NAME = 'base'.freeze
   DEFAULT_LOCALE = 'en'.freeze
+  MAX_BODY_LENGTH = 256.kilobytes
   CONTENT_FOR_LAYOUT_PATTERN = /\{\{\s*content_for_layout\s*\}\}/
 
   enum :locale, LANGUAGES_CONFIG.map { |key, val| [val[:iso_639_1_code], key] }.to_h, prefix: true
@@ -34,6 +35,7 @@ class EmailTemplate < ApplicationRecord
             if: :installation_scoped?
   validates :name, uniqueness: { scope: %i[account_id template_type locale], conditions: -> { where(inbox_id: nil) } }, if: :account_scoped?
   validates :name, uniqueness: { scope: %i[inbox_id template_type locale] }, if: :inbox_scoped?
+  validates :body, length: { maximum: MAX_BODY_LENGTH }
   validate :validate_inbox_account
   validate :validate_liquid_body
   validate :validate_layout_slot, if: :layout?