refactor: simplify CnameGenerator by using custom_cname directly

Replace generate_cname call with direct custom_cname usage in cdn_base_postfix
to reduce method indirection, addressing CodeRabbit review suggestion.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vipulnsward

.claude/settings.local.json

@@ -0,0 +1,15 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(node:*)",
+      "Bash(npm:*)",
+      "WebFetch(domain:github.com)",
+      "Bash(gh pr view:*)",
+      "Bash(gh pr diff:*)",
+      "WebFetch(domain:docs.anthropic.com)",
+      "Bash(git fetch:*)",
+      "Bash(git checkout:*)"
+    ],
+    "deny": []
+  }
+}

lib/uploadcare/cname_generator.rb

@@ -13,7 +13,7 @@ class << self
       def cdn_base_postfix
         @cdn_base_postfix ||= begin
           uri = URI.parse(Uploadcare.config.cdn_base_postfix)
-          uri.host = "#{generate_cname}.#{uri.host}"
+          uri.host = "#{custom_cname}.#{uri.host}"
           uri.to_s
         rescue URI::InvalidURIError => e
           raise Uploadcare::Exception::ConfigurationError, "Invalid cdn_base_postfix URL: #{e.message}"