selfhost(dashboard): dead-letter queue management UI

[JSONbored]

Part of #982.

Self-hosters need a way to inspect, replay, and purge dead-letter jobs from the dashboard — today they require direct SQLite/Postgres access.

Scope:

• REST endpoints: GET /api/selfhost/queue/dead (paginated), POST /api/selfhost/queue/dead/:id/replay, DELETE /api/selfhost/queue/dead/:id, DELETE /api/selfhost/queue/dead (purge all)
• Dashboard route: Dead-Letter Queue table with job ID, payload type, attempts, last error, created/died timestamps
• Replay button per row + bulk purge with confirmation
• Gate behind SELFHOST_API_SECRET auth (same as existing local API routes)
• SQLite and Postgres adapters both expose the same dead-queue interface