support: add a neutral system author for auto-created tickets (no clerk-assignment, no customer notification)

[TaprootFreak]

Context

PR #4182 makes the RealUnit API auto-open a support ticket when an Aktionariat registration forward fails (status ManualReview — there is no retry cron, a human must re-forward). The ticket is created via SupportIssueService.createIssueInternal.

For the first message we had to pick a message author. We chose CustomerAuthor as a pragmatic workaround, because it is the only value that leaves the ticket unassigned / awaiting staff without side effects.

Problem

author is not a neutral label — createMessageInternal (support-issue.service.ts) branches on it:

if (dto.author !== CustomerAuthor) {
  issue.setClerk(dto.author);                                 // assigns the ticket to a "clerk" named after the author
  supportIssueNotificationService.newSupportMessage(entity);  // notifies the customer "support replied"
}

There is no neutral system author: only Customer and AutoResponder exist, and AutoResponder still hits this branch. So a semantically correct author for a system-opened ticket (e.g. 'System' / 'Api') would:

1. assign the ticket to a phantom clerk named after the author → it looks handled and can drop out of the unassigned triage queue, and
2. push a misleading "support replied" notification to a customer who never opened a ticket.

Using CustomerAuthor avoids both, but attributes the auto-generated first message to the customer, which is semantically wrong (the message text makes the automatic origin clear, so it is acceptable for now).

Proposal

Introduce a real neutral system author and make createMessageInternal treat it as neither a customer nor a clerk reply:

• Add a SystemAuthor = 'System' constant next to CustomerAuthor / AutoResponder in support-message.entity.ts.
• In createMessageInternal, only assign a clerk + fire newSupportMessage for a real clerk author — i.e. skip both side effects when the author is SystemAuthor (leave the ticket unassigned / awaiting staff, no customer notification).
• Switch the RealUnit forward-failure ticket (and any future system-opened tickets) to SystemAuthor so the first message is correctly attributed to the system.

Acceptance criteria

• A ticket whose message author is SystemAuthor is left unassigned (no phantom clerk), awaiting staff (Pending), and triggers no customer "support replied" notification.
• Existing behaviour for Customer and real-clerk authors is unchanged.
• Unit tests cover the new author branch.

Notes

• Shared across all tenants (DFX + RealUnit) — needs care and its own tests; this is why it was deferred out of PR feat(realunit): open a support ticket and surface manual-review state when the Aktionariat forward fails #4182 rather than bundled in.
• Affected: support-message.entity.ts, support-issue.service.ts (createMessageInternal), plus tests.