Phase 17: a script to move the local user's writing onto a real account

The destination is an OIDC subject id, which the app cannot know — it
belongs to the identity provider — so this runs deliberately, with Petal
stopped and a backup taken, rather than as a startup migration. documents,
tags, vocab_words and images carry user_id directly; versions, suggestions
and tag assignments hang off their parents and follow, which is why it has
to be one transaction with foreign keys off. Sessions for the old identity
are deleted rather than moved: a session is proof someone signed in, and
nobody ever signed in as 'local'.

Dry run by default, VACUUM INTO backup first, and it verifies every row it
expected to move actually moved — and that the source is left owning
nothing — before committing.

The 'is the app stopped?' guard took two attempts. BEGIN EXCLUSIVE, the
obvious check, sails past a running-but-idle Petal because in WAL mode it
only conflicts with another writer, which is precisely the case worth
catching. PRAGMA locking_mode = EXCLUSIVE conflicts with any connection at
all, since it locks the shared-memory index every WAL reader maps.

Sequencing this also turned up a crash waiting to happen: the image
backfill claims unowned files for 'local', which no longer exists after a
migration, and the resulting foreign-key error is fatal inside images.New.
Petal would have crash-looped the first time it started on a migrated
database. It now skips a missing owner, which costs nothing — the
migration moves the image rows itself.

Claude-Session: https://claude.ai/code/session_016y6gyuHkQXPiEuW8RGQyua

internal/images/handler_test.go

@@ -209,6 +209,13 @@ func TestBackfillClaimsExistingFiles(t *testing.T) {
 	if _, err := New(dir, database.DB, "bob"); err != nil {
 		t.Fatalf("second backfill: %v", err)
 	}
+
+	// And an owner who no longer exists — which is what the `local` account
+	// becomes once it has been migrated onto a real one — must be skipped, not
+	// turned into a foreign-key error that takes startup down with it.
+	if _, err := New(dir, database.DB, "nobody-at-all"); err != nil {
+		t.Fatalf("backfill for a missing owner should be a no-op, got: %v", err)
+	}
 	var owners int
 	if err := database.QueryRow(`SELECT COUNT(*) FROM images WHERE name = ?`, orphan).Scan(&owners); err != nil {
 		t.Fatal(err)