P2.3: Migration runner adoption (DbUp/grate) with bash scripts as baseline

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Migrated from the pre-publish internal tracker (gitea-#253, opened 2026-07-16).

📄 Plugin-readiness session 2026-07-16 — docs/architecture/roadmap.md item P2.3 (gap: M5, ADR D5). ADR-0001 accepted (D1 in-process compile-time, D4 db-per-tenant lean).

Evidence: Hand-idempotent bash only (launch.sh:34-42); no ordering guarantees beyond filename glob, no down-migrations, no slot for module-shipped schema.

What: Adopt a runner when the first module ships tables (D5); bash + schema_version stamp (P1.3) suffices until then.

Size: M

What breaks if deferred: Module schema delivery has no slot; fleet upgrades can't reason about schema state.

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Repository review 2026-09-10 (R05), re-checked on v5.8.2: still no applied-migration ledger or checksum in migrate.sh; setup.sh:1147 infers currency from the newest marker table, which cannot prove that every earlier column/data migration landed (db/47 is column-only and already degrades that check). Acceptance to carry into this issue: fresh install and upgrade-from-previous-tag converge to the same schema; a failed migration names itself and leaves a safe retry; concurrent launchers cannot race; editing an applied migration is detected. The cheap launch-time fix is separate: gh-#770.