Cascade-archive notification parity: dispatch per-child WATCH_ARCHIVED notifications

[gregoryfoster]

Context

CR follow-up from #161 (item 7 in the round-1 review of docs/plans/2026-05-17-watched-item-crud-ui-plan.md).

The single-Watch danger-zone archive route at src/dashboard/routes.py:766-782 fires `_dispatch_archive_notification` for the archived Watch — operators get a notification when a Watch is archived.

The bulk WatchedItem archive (via src/api/routes/watched_items.py:80-118, called from both the API and the dashboard wrapper) cascades `is_active=False, is_archived=True` onto every child Watch and emits a `WATCH_ARCHIVED` audit row per child — but does not dispatch notifications. Operators watching for archive events on those Watches won't be notified.

This was a deliberate v1 omission to avoid notification floods on bulk archive (e.g. a WatchedItem with 30 children would fire 30 notifications), but it leaves operators with an inconsistent signal: single-Watch archive → notified; bulk WatchedItem archive → silent.

Scope

Pick a strategy:

1. Dispatch per-child notifications inline during cascade — accept the flood risk; matches the single-Watch path.
2. Emit one aggregate notification at the WatchedItem level ("Archived WatchedItem X — cascaded to N child Watches") — quieter, requires a new event type.
3. Suppress per-child notifications during cascade, document the gap — keep current behavior, add explicit note in operator docs.

Recommend (2) — one event per cascade, with the child count in the payload. Requires:

• New `WatchEventType.WATCHED_ITEM_ARCHIVED` (or similar).
• Dispatch from `archive_watched_item` after the cascade loop.
• A WatchedItem-level notification config / template might be required, or hijack the parent's notification dispatch path.

Acceptance

• Decide and document the strategy.
• If implementing (1) or (2): operators receive a notification when a WatchedItem is archived (and its children are cascaded), matching the per-Watch archive UX.
• Audit log unchanged from current behavior (parent + per-child `WATCH_ARCHIVED` audits stay).

Related

• WatchedItem CRUD UI — edit shared defaults from the dashboard #161 — WatchedItem CRUD UI (parent issue; round-1 CR observation)

[gregoryfoster]

WatchedItem cascade-archive shipped in #161 (commit a008f0a) is the call site this issue's notification gap applies to. archive_watched_item in src/api/routes/watched_items.py iterates child Watches and flips is_archived + emits per-child WATCH_ARCHIVED audit but does not dispatch notifications.