ALL-1367: Fix monitor detail legacy transition context crash

PAUSED PER SEPH (thread 1780406984.073309 at 13:38:23Z): "this is the ONLY monitor created before severity was required. Can we get delete / archive actions working first and I can archive it?" Plan pivot was: ship archive UI first (now done — PR #11526 merged 14:22Z), let Seph archive cmpqbihdb13qdms1t05b0z1xg, then RE-EVALUATE whether the reader-side fallback patch is needed at all. A heartbeat already shipped the patch as DRAFT PR #11529 by mistake (replayed the old plan from the original description). Do NOT re-spawn this work without explicit direction from Seph. The PR is parked in draft pending his call: close as unneeded, or keep as defense-in-depth. ORIGINAL CONTEXT (kept for reference, not for execution): Hotfix for prod monitor detail load failure reported by Seph in Slack thread 1780406984.073309. Root cause: older manual monitor cmpqbihdb13qdms1t05b0z1xg has MonitorTransition.context JSON without severity; current mapTransition parses context with severity required, so detail query fails when transitions are requested. Original plan was to implement reader-side fallback to paramSnapshot.severity for legacy context — Seph asked to ship archive-first instead.