Merge two support tickets into one thread

Imported from GitHub issue El-SaMa/oma#133 by @El-SaMa.

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User story

As an admin, I want to merge two support tickets, so that one thread holds the conversation when the same customer opens a second ticket about the same problem.

Background

There is no merge action. A second ticket stays open beside the first, and a reply on one does not close the other.

Lived: Coocos #LH-1014 and #LH-1024, same sender, same mail problem.

Acceptance criteria

• From a ticket, staff can pick another open ticket for the same organization or the same customer email.
• Messages from the source ticket appear on the target ticket in time order.
• The source ticket is marked merged and links to the target. It is no longer waiting on an agent.
• The customer receives one notice, sent only to tickets.customer_email on the ticket that stays open. Do not mail billing contacts or every address on the org.
• Internal notes stay internal.
• A later public reply still goes to that one opener address.

Out of scope

• Merging tickets across organizations.
• Automatic merge.