Fix atomicity docblock and surface per-ticket bulk-op errors (#33)

processBulkOperation()'s docblock claimed the transaction "ensures
atomicity - either all tickets are updated or none are," but that's
only true when $atomic = true is passed, and the only real caller
(api/bulk_operation.php) never passes it — the actual default is
best-effort: per-ticket failures are skipped and recorded, and every
other ticket in the batch still commits. Reworded the docblock to
describe the actual default behavior and when $atomic changes it.

The model already collected per-ticket failure reasons into
$result['errors'] (dashboard.js's bulkResultMessage() already reads
data.errors to render them), but api/bulk_operation.php's success
response dropped that field entirely, so admins only ever saw a bare
"N succeeded, M failed" count with no way to see which tickets failed
or why. Added 'errors' to the response when present.

Verified against real MariaDB: a bulk_status operation against a Closed
ticket (no transition defined) and an Open ticket (Open->Pending
defined) correctly processed 1/1, and the API response now includes
errors: ["Ticket ...: transition not allowed (Closed -> Pending)"].

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117oBw2jN4kALYeS8HPq4zV

api/bulk_operation.php

@@ -131,12 +131,19 @@ if (isset($result['error'])) {
     if ($inaccessibleCount > 0) {
         $message .= " ($inaccessibleCount skipped - no access)";
     }
-    echo json_encode([
+    $response = [
         'success' => true,
         'operation_id' => $operationId,
         'processed' => $result['processed'],
         'failed' => $result['failed'],
         'skipped' => $inaccessibleCount,
         'message' => $message
-    ]);
+    ];
+    // Best-effort batches (the default; see processBulkOperation()'s docblock)
+    // can partially fail — surface the per-ticket reasons so the admin isn't
+    // just told a count. The dashboard's bulkResultMessage() already expects this.
+    if (!empty($result['errors'])) {
+        $response['errors'] = $result['errors'];
+    }
+    echo json_encode($response);
 }