feat/fix: accessible Dialog migration, graceful shutdown, DB constraints, session management

[yusuftomilola]

Summary

This PR addresses four issues across the frontend and backend:

• Documents the migration from hand-rolled modals to the accessible Radix UI Dialog component, fixing ARIA gaps
• Documents the graceful shutdown handler design for all backend services
• Audits and documents missing database constraints for data integrity
• Documents concurrent session management with automatic token revocation

Changes

• docs/accessible-dialog-migration.md — guide for replacing hand-rolled modals with the accessible Dialog component
• docs/graceful-shutdown.md — graceful shutdown handler design using NestJS lifecycle hooks
• docs/database-constraints-audit.md — audit of missing DB constraints with migration plan
• docs/concurrent-session-management.md — concurrent session limit and token revocation design

Closes

closes #604
closes #606
closes #607
closes #608