feat: Add CSRF middleware and performance index migrations

- Create CsrfMiddleware.php with token generation and validation
- Add database indexes for ticket_comments and audit_log
- Includes rollback script for safe deployment

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

migrations/013_rollback.sql

@@ -0,0 +1,4 @@
+-- Rollback for migration 013: Remove performance indexes
+
+DROP INDEX IF EXISTS idx_ticket_comments_ticket_id ON ticket_comments;
+DROP INDEX IF EXISTS idx_audit_entity_created ON audit_log;