Add audit log plugin

[owjs3901]

Summary

• persist an opt-in Event::AUDIT marker in the transactional event outbox
• add the administrator-only audit-log plugin with exact filtering, pagination, and typed retention settings
• mark memo create/update/delete events for audit history while preserving pending delivery rows during retention

Validation

• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace (101 passed)
• RUSTDOCFLAGS=-D warnings cargo doc --workspace --no-deps
• bunx oxlint
• bun x tsc --noEmit in packages/app, apps/example-app, and every plugin
• cargo run -p yeollin-cli -- build --skip-backend

Browser QA

• signed in as a seeded administrator and created a memo through the real UI
• verified memo.created payload rendering, exact-name empty state, clear, refresh, and disabled pagination
• saved retention from 90 to 30 days on the generated settings page and verified the audit summary updated
• visually checked the corrected two-column desktop layout
• warmed fresh tab: 0 console errors and 0 HTTP failures
• stopped the dev process tree, confirmed ports 3000/3001 clear, and restored the prior QA database