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.

database/migrations/2026_08_29_000001_a_system_note_is_a_ticket_message_source.php

@@ -0,0 +1,66 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\DB;
+
+/**
+ * MERGE AND SPLIT WROTE A `source` THE COLUMN DID NOT ACCEPT.  [central-ticketing] #415
+ *
+ * Both actions write an audit note into `ticket_messages` with `source = 'system'`,
+ * and the column was created as `enum('web','email')`. Under strict mode the insert
+ * throws `Data truncated for column 'source'` INSIDE the transaction, so nothing
+ * commits: no merge, no split, no partial state, and a generic 500 page. Every
+ * guard around them worked — a missing reference, a self-merge and an absent
+ * `source_number` were all handled correctly — so only the happy path was broken,
+ * which is why it survived.
+ *
+ * ── THE WHOLE DOMAIN, NOT THE ONE IN THE TRACE ──────────────────────────────
+ *
+ * The trace named `'system'`. Widening to accept only that would be a guess about
+ * how many more there are, so every value written to this column anywhere in the
+ * platform was enumerated first:
+ *
+ *   'web'     TicketMessageController::store(), the demo seeder
+ *   'email'   the demo seeder (no inbound-mail ingester exists yet)
+ *   'system'  TicketController::merge() and ::split(), the three notes
+ *
+ * `split()` also copies `$msg->source` verbatim when duplicating a thread, which
+ * introduces no fourth value. So the set is closed at three and this runs once.
+ *
+ * ── WHY AN ENUM AND NOT A VARCHAR ───────────────────────────────────────────
+ *
+ * `tickets.status` went the other way — `2026_04_17_000002` deliberately widened it
+ * from an enum to `varchar(50)` — and the difference is the test worth writing down:
+ * statuses became OPERATOR-DEFINABLE rows in `ticketing_statuses`, so no enum could
+ * ever hold them. `source` is the opposite: all three values are set by code, an
+ * operator has no way to add a fourth, and the enum is what makes a typo'd source a
+ * loud failure rather than a silently unfilterable row.
+ *
+ * The write side is bounded too — `TicketMessage::system()` is now the only place a
+ * system note is created, so the three scattered `'source' => 'system'` literals that
+ * caused this cannot reappear one at a time.
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        DB::statement(
+            "ALTER TABLE ticket_messages
+             MODIFY COLUMN source ENUM('web','email','system') NOT NULL DEFAULT 'web'"
+        );
+    }
+
+    public function down(): void
+    {
+        // Rewrite the rows first: narrowing an enum that still holds 'system' turns
+        // those rows into the empty string rather than failing, which would leave the
+        // merge and split notes present but unattributable. 'web' is the honest
+        // fallback — they were written by a person pressing a button in the app.
+        DB::table('ticket_messages')->where('source', 'system')->update(['source' => 'web']);
+
+        DB::statement(
+            "ALTER TABLE ticket_messages
+             MODIFY COLUMN source ENUM('web','email') NOT NULL DEFAULT 'web'"
+        );
+    }
+};