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

models/BulkOperationsModel.php

@@ -106,12 +106,18 @@ class BulkOperationsModel
     /**
      * Process a bulk operation
      *
-     * Uses database transaction to ensure atomicity - either all tickets
-     * are updated or none are (on failure, changes are rolled back).
+     * Runs the whole batch inside one database transaction, but by default
+     * ($atomic = false, which is what api/bulk_operation.php uses) that
+     * transaction is always committed: a per-ticket failure (e.g. a
+     * disallowed workflow transition) is recorded in $failed/$errors and
+     * skipped, while every other ticket in the batch still succeeds. This
+     * is a best-effort batch, not an all-or-nothing one — set $atomic to
+     * true to roll back the entire batch when any ticket fails.
      *
      * @param int $operationId Operation ID
      * @param bool $atomic If true, rollback all changes on any failure
-     * @return array Result with processed and failed counts
+     * @return array Result with processed/failed counts and an errors[] list of
+     *               per-ticket failure reasons (surfaced to the admin by the caller)
      */
     public function processBulkOperation($operationId, bool $atomic = false)
     {