feat(backup): open sqlite → pg .picpeak restore as the supported upgrade direction (#1041) (#1043)

Reshaped onto main after #1039 landed the coercion engine
(typedColumnsFor / epochToIso / coerceForTargetEngine) — this PR is now
only the policy delta on top of it:

- validateManifest: replace the CLI-only allowEngineSwitch opt-in with a
  direction rule — sqlite → pg allowed (upload UI and CLI alike),
  pg → sqlite refused with a message naming the supported direction
- importFromPicpeak: derive crossEngine from the manifest's engine
  (absent field = target engine, the exact pre-change behavior), log it,
  return it; route passes it through
- scripts/migrate-sqlite-to-postgres.js: rely on the shared gate, drop
  the flag
- restore card: direction stated in the intro, cross-engine notice after
  a converting restore; both strings in en.json + de.json; removed the
  orphaned settings.backup.picpeak locale node (unreferenced, stale copy)
- picpeakCrossEngine.test.js: direction policy, epochToIso (ms, seconds,
  numeric strings), coerceForTargetEngine units, plus
  PICPEAK_PG_TEST_URL-gated real-Postgres stored-value assertions

Co-authored-by: Paul Nothaft <[email protected]>

backend/scripts/migrate-sqlite-to-postgres.js

@@ -201,9 +201,9 @@ async function phaseImport(archivePath) {
   const { importFromPicpeak } = require('../src/services/picpeakImportService');
   // No currentAdminId: this is a CLI, there is no operator session to preserve.
   // The SQLite install's own admin accounts come across with everything else.
-  // allowEngineSwitch: moving between engines is the whole point here. The
-  // upload/restore UI keeps refusing it.
-  const summary = await importFromPicpeak({ picpeakPath: archivePath, allowEngineSwitch: true });
+  // sqlite → pg is allowed by validateManifest's direction policy (#1041) —
+  // the same gate the upload/restore UI uses, no separate opt-in flag.
+  const summary = await importFromPicpeak({ picpeakPath: archivePath });
   return JSON.stringify(summary || {});
 }