Reject duplicate workflow transitions with a clear error (#62)

status_transitions already has a DB-level UNIQUE KEY on
(from_status, to_status), so a genuine duplicate pair was never
actually possible to insert — but hitting that constraint raw
surfaced as an opaque "An internal error occurred" to the admin
instead of a clear message, since manage_workflows.php only validated
from_status !== to_status before attempting the insert/update.

Added an explicit existence check before insert/update in both the
POST and PUT handlers (excluding the row's own ID on update), so the
common case — an admin re-adding or renaming into a pair that already
exists — gets a specific 409 with the conflicting pair named, instead
of a generic 500. Also added ORDER BY transition_id to
WorkflowModel::getAllTransitions() as a defense-in-depth backstop:
since it collapses rows into a PHP array keyed by
[from_status][to_status] with no defined winner otherwise, if the DB
constraint were ever weakened or bypassed, this at least makes which
row wins deterministic (most recently created).

Verified against a real running server + real MariaDB: creating a
duplicate active pair, a duplicate inactive pair, and updating a
different row into an existing pair are all correctly rejected with
the friendly message; updating a row to keep its own existing pair
succeeds; and a genuinely different pair still creates normally.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lhz7pGMaoTfL5sdYS5XiKv

models/WorkflowModel.php

@@ -31,9 +31,15 @@ class WorkflowModel
             return $cached;
         }
 
+        // ORDER BY makes which row wins deterministic (most recently created,
+        // by transition_id) in the pathological case where two active rows
+        // exist for the same (from_status, to_status) pair — manage_workflows.php
+        // now rejects creating that duplicate going forward, but this is a
+        // defense-in-depth backstop against any duplicate already in the DB.
         $sql = "SELECT from_status, to_status, requires_comment, requires_admin
                 FROM status_transitions
-                WHERE is_active = TRUE";
+                WHERE is_active = TRUE
+                ORDER BY transition_id ASC";
         $result = $this->conn->query($sql);
 
         if (!$result) {