feat: add pipeline observability and catch-up status

[DevSecNinja]

Why

GitHub scheduled workflows are best-effort and can be delayed. Wazzup should make it obvious whether the hourly pipeline is healthy, delayed, skipped by cadence, or failing.

Scope

• Publish run metadata in generated data: last attempted run, last successful run, provider, briefing kind, source count, generated item count, and status.
• Surface pipeline health in the PWA footer or source-health area without making the UI noisy.
• Distinguish source failures from AI/provider failures.
• Add a documented/manual catch-up workflow path for missed or delayed scheduled runs.
• Consider a lightweight alert path when a scheduled run is stale for too long.

Acceptance criteria

• The app can show when the latest briefing was generated and whether the pipeline is stale.
• A missed/delayed schedule is distinguishable from a failed generation.
• Manual catch-up steps are documented.
• Tests cover the published run-status data shape.