feat(backup): open sqlite → pg .picpeak restore as the supported upgrade direction (#1041) (#1059)

Stable backport of #1043 (main: 8809564a).

sqlite → pg restore is allowed for anyone holding backup.restore, from the
upload UI and the CLI alike, gated by the manifest-direction rule in
validateManifest. pg → sqlite stays refused, with an error naming the
supported direction. allowEngineSwitch is removed rather than kept alongside:
one gate, no way to drive the refused direction.

Two resolutions were needed against stable rather than a clean cherry-pick,
both from known main/stable divergences:

  - replaceAllTables has no roleSnapshot parameter on this branch, so the call
    keeps stable's 4-arg signature while taking the derived { crossEngine }.
  - resyncSequences was guarded by `if (allowEngineSwitch)`, which this change
    removes — leaving an undefined reference. It now runs unconditionally,
    matching main. That also closes a stable-only gap: a same-engine pg → pg
    restore previously left identity sequences stale, so the next natural
    insert collided on the primary key.

Also exports resyncSequences (the function already existed here, main already
exports it) so the cross-engine suite can drive the post-restore fixup.

Verified on this branch: all four picpeak suites green on SQLite, and 20/20
against a real Postgres 15 with the PICPEAK_PG_TEST_URL-gated cases executing.

Co-authored-by: Paul Nothaft <paul@MacStudio-von-Paul.local>

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 || {});
 }