Add persistence, multi-repo watch list, and background polling0.3.0

Closes #1-#5 (milestones 0.2.0, 0.3.0).

- Flat JSON persistence (PersistedState/PersistenceStore) in Application
  Support, chosen over SwiftData since the dataset is small and the
  existing model types are plain Codable value types passed across actor
  boundaries.
- Repo watch list, per-event seen-state, and last-fetch timestamps are
  now persisted instead of living only in memory.
- Configurable minimum severity filter, applied from a cached raw fetch
  so changing it doesn't require a network round-trip.
- Multi-repo support: SecurityEventStore now loops over a persisted watch
  list instead of one hardcoded repo, with an in-popover UI (gear button)
  to add/remove repos.
- Background polling every 15 minutes, layered on top of the existing
  refresh-on-open and manual refresh, so the feed stays fresh even while
  the popover is closed.

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