Follow-ups from #105: notification tab, per-doc confidence weighting, dashboard surfacing

Follow-up to #105 / PR #106. The feedback→confidence loop shipped in #106; these remaining #105 pieces are tracked here:

In-platform notification surface

• New Prisma Notification model (type=ESCALATION, ticketId, source, sourceUrl, assignee, readAt — global read state).
• Producer in the ESCALATION job handler (packages/outpost/queue/src/handlers/escalation.ts) — one place covers Discord/Slack/Teams/command.
• Notifications tab in the Outpost web sidebar (apps/web/src/components/sidebar.tsx) + unread bell; /notifications route + list/mark-read API.
• Design already brainstormed (see the "In-platform escalation notifications" design discussion on #105).

Confidence weighting — deeper slices

• Per knowledge-source / doc reputation (requires persisting the search-result docs behind each AI Message).
• Per topic / keyword cluster weighting.

Dashboard surfacing

• Show the current global calibration factor + 👎 clusters (knowledge-gap view) in analytics.

Start with the design discussion on #105 and the ESCALATION handler in packages/outpost/queue/src/handlers/escalation.ts, then inspect apps/web/src/components/sidebar.tsx and the related web routing and API entry points. Done means the notification model, producer, notifications UI and read API, deeper confidence weighting, and dashboard calibration and knowledge-gap views are implemented.