fix(coach): salvage Coach preferences field by field

parseCoachPrefs parsed the stored blob as a whole and returned the
defaults on any invalid field. A lookback value this version does not
know therefore dropped the metric exclusions with it. Each field now
falls back on its own, and the exclusion and cluster lists keep their
known entries, so drift narrows what the Coach reads instead of widening
it.

The deploy runbook's rollback section notes the one case this cannot
cover: v1.40.1 and earlier still parse the blob whole, so an account that
saved the new "Last year" lookback loses its exclusions there.

docs/ops/deploy.md

@@ -78,6 +78,12 @@ roll back:
 dev --create-only --name vX_Y_Z_rollback_<col>`), edit the SQL,
    commit, and re-deploy. Never `migrate resolve --rolled-back` against
    production unless you've taken a fresh `pg_dump` first.
+4. Rolling back from v1.40.2 or later to v1.40.1 or earlier: an account
+   that saved the Coach lookback "Last year" reads its whole Coach
+   preference blob as defaults on the older version, metric exclusions
+   included, until the setting is saved again. Ask affected accounts to
+   pick another lookback before the rollback, or re-save their exclusions
+   after it.
 
 ## Catalog templates