Inactive-community rail: count marker-only @channel/@here mentions

Deferred finding (F8) from the adversarial review of #3233 (per-channel notification settings, #3160). Depends on #3197 (@channel/@here, #3146).

`communityUnreadObserver` builds the numeric mention count for inactive communities from a `#p` filter only. Once #3197 lands, marker-only `["notify","channel"]` events are mention-tier in the active community but invisible to the inactive-community rail count — the rail shows a dot and a zero mention count for the same unread event the active community classifies as high priority.

Fix after #3197 merges: extend the observer query (or reuse #3197's mentions-feed source) so notify-marker events enter the rail count, gated by the reader's per-channel level + broadcasts opt-out exactly like the active-community ladder in `notifyDecisionForEvent`.

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