fix(db): harden migration recovery snapshots

[Bl0ck154]

Stacked on PR #12 (integrate/upstream-db-safety). Production is not changed by this draft PR.

Included

• upstream fix(db): harden migration recovery snapshots diegosouzapw/OmniRoute#12435 migration recovery hardening
• content-addressed pre-migration snapshots for existing DBs
• schema-state tracking before migration writes
• stronger physical-schema safety checks
• backup/data-dir retention and eval/test isolation updates required by the migration safety path

Adaptation to our branch

• preserved our existing safe-init wrapper around DB initialization
• passed databaseExistedBeforeInitialization into the new migration runner without replacing our newer startup logic
• kept our actual 28-entry RENAMED_MIGRATION_COMPATIBILITY table instead of importing four unrelated later release-tip renames; tests were adjusted to our real table ordering
• did not pull unrelated plugin-directory docs from the newer release tip

Validation

• 40/40 focused migration/backup/data-dir tests pass
• npm run typecheck:core: pass
• npm run check:db-rules: pass
• npm run check:env-doc-sync: pass
• eslint on the migration/core/data-path files touched here: pass
• git diff --check: pass