feat(web): maintenance windows in the alerts hub - #2345

🟢 Confidence 10/10 · safe to merge. Paging, public-id encoding and the changed-fields diff all check out, and each main flow is pinned by the new web, API and backend tests.
_{no findings · tests covered · risk medium}

Adds a Maintenance tab, create/edit dialog and occurrence links for maintenance windows, plus a maintenance_window_id filter on GET /v2/alerts/silences. Self-contained, well tested, and the earlier badge-overflow defect is fixed here; safe to merge.

• Maintenance tab lists windows with schedule, coverage and next occurrence
• maintenance-dialog creates windows and PATCHes only the fields an edit changed
• GET /v2/alerts/silences filters by maintenance_window_id in SQL, scoped to the org
• New rowIcon in packages/views; on-call and chains rows reuse it

Fixed since the last review

• ✅ F6 · Managed by ${windowName} badge never truncates, so a long window name overflows the row

_{ea1b118 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}