feat: Add unread count filters feature flag (1/6) (#14885)

## Description

Adds the account-level `unread_count_for_filters` feature flag as the
dark-launch gate for filtered sidebar unread counts. This reuses the
deprecated `quoted_email_reply` flag slot, resets the reused bit for
existing accounts, and removes stale defaults so new accounts do not
reference the old flag.

This also adds the feature where we are now calculating the unread counts for built in filters like mentions, participating and unattended along with unread count for saved filters/folders.

Closes
[CW-7262](https://linear.app/chatwoot/issue/CW-7262/unread-counts-for-filters-folders)

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

db/migrate/20260629000000_repurpose_quoted_email_reply_flag_for_unread_count_for_filters.rb

@@ -0,0 +1,22 @@
+class RepurposeQuotedEmailReplyFlagForUnreadCountForFilters < ActiveRecord::Migration[7.1]
+  def up
+    # The quoted_email_reply flag (deprecated) has been renamed to unread_count_for_filters.
+    # Disable it on any accounts that had quoted_email_reply enabled so the repurposed
+    # flag starts in its intended default-off state.
+    Account.feature_unread_count_for_filters.find_each(batch_size: 100) do |account|
+      account.disable_features(:unread_count_for_filters)
+      account.save!(validate: false)
+    end
+
+    # Remove the stale quoted_email_reply entry from ACCOUNT_LEVEL_FEATURE_DEFAULTS.
+    # ConfigLoader only adds new flags; it never removes renamed ones.
+    # Leaving it would cause NoMethodError in enable_default_features when
+    # creating new accounts (feature_quoted_email_reply= no longer exists).
+    config = InstallationConfig.find_by(name: 'ACCOUNT_LEVEL_FEATURE_DEFAULTS')
+    return if config&.value.blank?
+
+    config.value = config.value.reject { |feature| feature['name'] == 'quoted_email_reply' }
+    config.save!
+    GlobalConfig.clear_cache
+  end
+end