Add configurable data retention workflow

[llbbl]

Closes #130

Summary

• Add typed retention policies for message archives, archived messages, CSP reports, and audit events.
• Implement dry-run, per-policy selection, archive delegation, and bounded deletion through retention:apply.
• Document cron scheduling, hot-cold storage, backups, and manual database maintenance.

Testing

• Focused retention tests: 9 tests, 31 assertions
• composer test:all: 227 tests, 900 assertions
• WebSocket tests: 7 tests, 60 assertions
• composer analyse
• composer cs-check
• pnpm check
• pnpm build
• pnpm e2e: 6/6 passed
• Independent PHP review: PASS
• Mutation testing was not run because it is manual-only.