Add performance, security, and reliability improvements

- Consolidate all 20 API files to use centralized Database helper
- Add optimistic locking to ticket updates to prevent concurrent conflicts
- Add caching to StatsModel (60s TTL) for dashboard performance
- Add health check endpoint (api/health.php) for monitoring
- Improve rate limit cleanup with cron script and efficient DirectoryIterator
- Enable rate limit response headers (X-RateLimit-*)
- Add audit logging for workflow transitions
- Log Discord webhook failures instead of silencing
- Fix visibility check on export_tickets.php
- Add database migration system with performance indexes
- Fix cron recurring tickets to use assignTicket method

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

migrations/001_add_indexes.sql

@@ -0,0 +1,48 @@
+-- Migration: Add Performance Indexes
+-- Run this migration to improve query performance on common operations
+
+-- Single-column indexes for filtering
+-- These support the most common WHERE clauses in getAllTickets()
+
+-- Status filtering (very common - used in almost every query)
+CREATE INDEX IF NOT EXISTS idx_tickets_status ON tickets(status);
+
+-- Category and type filtering
+CREATE INDEX IF NOT EXISTS idx_tickets_category ON tickets(category);
+CREATE INDEX IF NOT EXISTS idx_tickets_type ON tickets(type);
+
+-- Priority filtering
+CREATE INDEX IF NOT EXISTS idx_tickets_priority ON tickets(priority);
+
+-- Date-based filtering and sorting
+CREATE INDEX IF NOT EXISTS idx_tickets_created_at ON tickets(created_at);
+CREATE INDEX IF NOT EXISTS idx_tickets_updated_at ON tickets(updated_at);
+
+-- User filtering
+CREATE INDEX IF NOT EXISTS idx_tickets_created_by ON tickets(created_by);
+CREATE INDEX IF NOT EXISTS idx_tickets_assigned_to ON tickets(assigned_to);
+
+-- Visibility filtering (used in every authenticated query)
+CREATE INDEX IF NOT EXISTS idx_tickets_visibility ON tickets(visibility);
+
+-- Composite indexes for common query patterns
+-- These are more efficient than single indexes for combined filters
+
+-- Status + created_at (common sorting with status filter)
+CREATE INDEX IF NOT EXISTS idx_tickets_status_created ON tickets(status, created_at);
+
+-- Assigned_to + status (for "my open tickets" queries)
+CREATE INDEX IF NOT EXISTS idx_tickets_assigned_status ON tickets(assigned_to, status);
+
+-- Visibility + status (visibility filtering with status)
+CREATE INDEX IF NOT EXISTS idx_tickets_visibility_status ON tickets(visibility, status);
+
+-- ticket_comments table
+-- Optimize comment retrieval by ticket
+CREATE INDEX IF NOT EXISTS idx_comments_ticket_created ON ticket_comments(ticket_id, created_at);
+
+-- Audit log indexes (if audit_log table exists)
+-- Optimize audit log queries
+CREATE INDEX IF NOT EXISTS idx_audit_entity ON audit_log(entity_type, entity_id);
+CREATE INDEX IF NOT EXISTS idx_audit_user ON audit_log(user_id, created_at);
+CREATE INDEX IF NOT EXISTS idx_audit_action ON audit_log(action, created_at);