conversations: add Cc and Bcc recipients to support ticket replies

Is your feature request related to a problem?

When handling a support ticket over email, an agent often needs to loop in someone who is not the original requester: a colleague at the customer's company, or an internal coworker who does not use PostHog (for example someone who handles billing or sales). Today there is no way to add a Cc or Bcc recipient to a reply. A ticket's participants are only populated from the To and Cc headers of inbound email and are read-only, so an agent can only reach people who were already on the thread. Tagging a teammate works only for existing PostHog users, so it does not cover external contacts or non-PostHog colleagues.

Describe the solution you'd like

Let an agent add one or more Cc and/or Bcc recipients when composing a new ticket or replying to an existing one. The addresses should be included on the outbound email, and Cc recipients should be preserved on later replies in the thread the same way inbound participants are today.

Describe alternatives you've considered

• Tagging a PostHog team member via a private note. This only works for people who have PostHog accounts, so it cannot reach external or non-PostHog colleagues.
• Forwarding the email manually outside PostHog, which drops the context and breaks the ticket thread.

Additional context

An open draft PR is already implementing this: PostHog/posthog#73040 (add Cc/Bcc to support email replies). This issue tracks the capability for roadmap visibility. Requested by a customer via a support thread.

Debug info

• PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
• PostHog Hobby self-hosted with docker compose, version/commit: [please provide]
• PostHog self-hosted with Kubernetes (deprecated, see Sunsetting Kubernetes support), version/commit: [please provide]

Review the open draft PR PostHog/posthog#73040 first, since it already implements this capability. Compare its behavior with the issue requirements: agents can enter Cc/Bcc recipients on new and existing tickets, outbound messages include them, and Cc recipients persist on later replies.