`database.journal_mode: delete` can never self-apply to a store that is already WAL:
apply_wal_with_fallback deliberately never live-downgrades (#68545 — other gateway/cron/worker
connections may hold uncheckpointed WAL commits), so operators applying the containment for the
multi-writer corruption class saw one ERROR per process forever and the only escape hatch was an
undocumented hand-run PRAGMA on the file.
The new pre-DB `sessions set-journal-mode` verb is the sanctioned offline path: it refuses while ANY
foreign process holds the file or a sidecar (the same foreign_state_db_holders scan doctor/repair
admission uses, naming each PID), flips through _set_journal_mode_no_wait (busy_timeout=0, so an
opener appearing mid-way makes SQLite refuse instead of racing it), verifies header bytes 18/19,
and reminds the operator when config.yaml disagrees. `--db PATH` covers kanban.db / cron stores
that log the same ERROR. The never-live-downgrade invariant is untouched; the ERROR, doctor hints
and docs now name the command instead of the raw PRAGMA.
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