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/src/routes/adminBackup.js

@@ -248,6 +248,7 @@ router.post('/picpeak/import', adminAuth, requirePermission('backup.restore'), p
       tables: result.tables,
       filesRestored: result.filesRestored,
       usesExternalMedia: result.usesExternalMedia,
+      crossEngine: result.crossEngine,
       sessionInvalidated: true,
     });
   } catch (error) {