Add ABOUTME comments to all Perl files

[perigrin]

Summary

Per coding standards in CLAUDE.md, all code files should start with a 2-line ABOUTME comment explaining what the file does. Currently 85 out of 130 Perl files lack this documentation.

Requirements

• Each comment line should start with "ABOUTME: " for easy grepping
• Keep to 2 lines maximum
• Describe what the file does, not its history

Scope

Missing ABOUTME comments in:

• 10 Controller files (Waitlist, AdminDashboard, TeacherDashboard, etc.)
• 60+ DAO files (core business logic classes)
• Core classes (Registry.pm, Event.pm, User.pm, Tenant.pm, etc.)

Implementation Approach

Break into 3 PRs:

1. Controllers and utilities
2. DAO classes
3. Core domain classes

Example Format

# ABOUTME: Manages waitlist entries and automatic progression when spots open
# ABOUTME: Handles waitlist position calculations and notification triggers