A system note is a ticket message source

Merge and Split both 500ed. The audit note each writes uses `source = 'system'`
and the column is `enum('web','email')`, so the insert threw inside the
transaction and nothing committed — which is why the operator saw no state change
at all rather than a half-done merge.

Widened to an enum rather than a varchar, and the reasoning is the same test that
made `tickets.status` a varchar in `2026_04_17_000002`: statuses became
operator-definable rows, so their domain is open. `source` is the opposite — all
three values are set by code, an operator can never add a fourth, and the enum is
what keeps a typo'd source loud instead of a silently unfilterable row.

`TicketMessage::system()` is now the only place a system note is written. Three
scattered literals is how this reopens.

A SECOND 500 ON THE SAME ACTION, unreported and one longer title away. `split()`
appended ' (split)' to a title the product validates at max:255, so splitting a
full-length title produced 263 characters and the identical hard failure. Bounded
at the write rather than widened, and the distinction is written down: widen when
the data is right and the column is wrong; bound when the column expresses a real
product rule. 255 is the product's own limit, so a 263-character title would be
un-editable forever — the edit form would bounce it every time.

And `split()` now honours its own comment. Line 682 has always said "exclude
system notes" while the code copied them; it never mattered because no system row
could exist. The moment the enum accepts them, splitting a merged ticket would
carry "Merged from ticket QAX-0005." onto a ticket that was never merged.

src/Models/TicketMessage.php

@@ -5,9 +5,20 @@ namespace Dashboard\Ticketing\Models;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Support\Facades\Auth;
 
 class TicketMessage extends Model
 {
+    /**
+     * Every value `ticket_messages.source` accepts, written down once.
+     *
+     * The column is an enum and this is the same list, so the two can be compared
+     * rather than remembered. #415 was three `'source' => 'system'` literals against
+     * a column that only knew 'web' and 'email' — the value existed in the code for
+     * as long as merge and split have, and nowhere in the schema.
+     */
+    public const SOURCES = ['web', 'email', 'system'];
+
     protected $fillable = [
         'ticket_id', 'user_id', 'author_email', 'body',
         'is_internal', 'source', 'email_message_id',
@@ -17,6 +28,33 @@ class TicketMessage extends Model
         'is_internal' => 'boolean',
     ];
 
+    /**
+     * THE ONLY WAY A SYSTEM NOTE IS WRITTEN.
+     *
+     * Merge writes one and split writes two, and before this they were three
+     * separate literals in one controller — which is how all three carried a
+     * `source` the column rejected, and how a fourth would have joined them.
+     *
+     * A system note is always internal: it is the audit trail of an agent action,
+     * addressed to whoever works the ticket next, and never to the submitter.
+     */
+    public static function system(Ticket|int $ticket, string $body): self
+    {
+        return static::create([
+            'ticket_id'   => $ticket instanceof Ticket ? $ticket->id : $ticket,
+            'user_id'     => Auth::id(),
+            'body'        => $body,
+            'is_internal' => true,
+            'source'      => 'system',
+        ]);
+    }
+
+    /** A note this class wrote, rather than a person or a mailbox. */
+    public function isSystem(): bool
+    {
+        return $this->source === 'system';
+    }
+
     public function ticket(): BelongsTo
     {
         return $this->belongsTo(Ticket::class, 'ticket_id');