fix(notifications): exclude the comment actor so they never self-notify

[yumike]

Problem

When a user commented on a page inside an entity they owned, they received a notification for their own comment whenever the owner was a group they belong to. The owner-side rw.comments event addressed the owning entity (the group), and Backstage's recipient resolver expanded that group back to its members — including the actor.

Root cause: the publisher excluded the actor only by literal ref (recipients.filter(ref => ref !== actorRef)), which a group owner ref can never match.

Fix

Forward the actor as excludeEntityRef on the notification recipients. Backstage's resolver drops the actor from the resolved set after expanding groups, covering group, nested-group, and direct-user owners alike — the field's documented purpose ("the currently logged-in user, for preventing sending notification of user action to him/herself").

With exclusion correct at delivery, the publisher's literal-ref pre-filter was redundant (excludeEntityRef is a strict superset), so it's removed: actor exclusion now has a single home. The publisher emits the raw section owner / thread participants (which may include the actor) and the resolver drops the actor.

Trade-off: self-comments and sole-participant threads now publish an event that resolves to zero recipients instead of being skipped locally — one publish + one notification POST of wasted work (no catalog expansion for the direct-user case).

Tests

• actorRef is now validated in the subscriber's shape guard (the sole basis for exclusion).
• Added regression coverage, including an owner==actor publisher test that fails if the literal filter is reintroduced.

Verification (e2e, dev app)

• Group member commenting on their own entity → no notification.
• Non-member commenting → member owners still notified.
• Owner commenting on a directly-owned entity → no notification.

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