feat(alerts): build escalation chains in the dashboard and see them on incidents - #2305

🟢 Confidence 8/10 · likely safe to merge. Held at 8 because 2 reviewed files were not read.
_{no findings · tests covered · risk medium · 2/3 new units observable −1 · 2 files not read · 1 fix not yet re-checked against the whole diff}

Adds escalation chains to the dashboard: a Routing-tab section with create/edit/delete, a per-tier picker in the rule editor, an escalation block on the incident rail, and a nullable escalation on v2 alert incidents filled from one batched run query per page. Safe to merge.

• listIncidentEscalations reads a page's latest runs in one query and v2 incidents expose escalation
• Routing tab gains the chains section, the chain dialog and a 409 delete naming its blockers
• Rule editor tiers get an escalation-chain picker beside their destinations
• Incident rail shows escalating, paused or finished, re-read once a step is due

Fixed since the last review

• ✅ F24 · A refused chain delete can name the wrong rules and miss the one blocking it

Production impact

_{f92b112 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}