feat: add configurable notification system

[yunhai-dev]

Summary

• add durable PostgreSQL notification state and transactional outbox delivery
• add configurable ServerChan administrator alerts and SMTP user quota alerts
• notify on channel/proxy incident lifecycles and user/API-key quota thresholds
• add protected notification worker, settings UI, secret masking, and focused tests

Validation

• bun test lib/notifications.test.ts
• bun test lib/channel-health.test.ts
• bun test lib/proxy-lifecycle.test.ts
• bun test lib/log-generator.test.ts
• bunx tsc --noEmit
• bun run build

Notes

• PostgreSQL schema initialization was not run locally because DATABASE_URL was not configured.
• All notification switches default to disabled.