Harden recurring cron, bulk delete, error handling; fix semgrep CI

Continued fixes from the multi-agent review:

- recurring tickets cron: now that the parse error is fixed the job runs,
  exposing two latent bugs. (1) next_run_at was only advanced after the
  full success path, so any failure (e.g. a NULL created_by passed to the
  non-nullable assignTicket() $assignedBy -> TypeError) left it in the past
  and re-created a duplicate ticket every cron cycle. Added an atomic
  claimForRun() (conditional UPDATE gated on still-due) called BEFORE
  creation, which also prevents overlapping runs from double-creating.
  (2) The cron used a raw mysqli with no utf8mb4, corrupting non-ASCII
  content; it now uses Database::getConnection(). Also guard the assignment
  so created_by NULL falls back to the assignee.
- bulk delete: attachment files were unlinked inside the DB transaction, so
  an atomic-mode rollback restored rows but the files were already gone.
  deleteTicket() can now defer file removal to the caller, and
  BulkOperationsModel deletes files only after a successful commit.
- UserModel: back-tick the `groups` column (reserved word on MySQL 8.0.2+).
- create_ticket_api.php: stop leaking raw DB/exception messages to callers;
  log server-side and return a generic error. (Also includes a pre-existing
  working-tree tweak that adds title to the manual-ticket dedupe hash.)
- CI: semgrep install failed under PEP 668; add --break-system-packages.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

models/UserModel.php

@@ -86,7 +86,7 @@ class UserModel
             $user = $result->fetch_assoc();
 
             $updateStmt = $this->conn->prepare(
-                "UPDATE users SET display_name = ?, email = ?, groups = ?, is_admin = ?, last_login = NOW() WHERE username = ?"
+                "UPDATE users SET display_name = ?, email = ?, `groups` = ?, is_admin = ?, last_login = NOW() WHERE username = ?"
             );
             $updateStmt->bind_param("sssis", $displayName, $email, $groups, $isAdmin, $username);
             $updateStmt->execute();
@@ -100,7 +100,7 @@ class UserModel
         } else {
             // Create new user
             $insertStmt = $this->conn->prepare(
-                "INSERT INTO users (username, display_name, email, groups, is_admin, last_login) VALUES (?, ?, ?, ?, ?, NOW())"
+                "INSERT INTO users (username, display_name, email, `groups`, is_admin, last_login) VALUES (?, ?, ?, ?, ?, NOW())"
             );
             $insertStmt->bind_param("ssssi", $username, $displayName, $email, $groups, $isAdmin);
             $insertStmt->execute();
@@ -300,7 +300,7 @@ class UserModel
             return $cached;
         }
 
-        $stmt = $this->conn->prepare("SELECT DISTINCT groups FROM users WHERE groups IS NOT NULL AND groups != ''");
+        $stmt = $this->conn->prepare("SELECT DISTINCT `groups` FROM users WHERE `groups` IS NOT NULL AND `groups` != ''");
         $stmt->execute();
         $result = $stmt->get_result();