fix(setup): correct month index and zero-pad database backup file names

Backup names were built inline from Date#getMonth, which is zero-indexed, so a
backup taken on 12 September 2026 was written as db_2026-8-12_20-35-56.sqlite.
No field was zero-padded either, giving names like db_2026-8-12_20-36-2.sqlite.

Extract formatBackupTimestamp into server/lib and use it from both places that
built the string: the backupDb helper in migrationsSqlite.ts and the inline copy
in the 1.0.0-beta9 setup script. Padding every field also makes the names sort
lexicographically in the order the backups were taken.

Adds tests covering both reported names, single-digit padding and sort order.
Reverting the helper to the old formula fails them with the exact name from the
report.

server/lib/backupFileName.ts

@@ -0,0 +1,28 @@
+/**
+ * Builds the timestamp segment of a database backup file name.
+ *
+ * `Date#getMonth` is zero-indexed, so building this inline produced names like
+ * `db_2026-8-12_...` for a backup taken on 12 September 2026. Every field is
+ * also zero-padded, which keeps the names unambiguous and makes them sort
+ * lexicographically in the order they were taken.
+ *
+ * @param date The moment the backup is being taken. Defaults to now.
+ * @returns A timestamp of the form `YYYY-MM-DD_HH-MM-SS`.
+ */
+export function formatBackupTimestamp(date: Date = new Date()): string {
+    const pad = (value: number): string => String(value).padStart(2, "0");
+
+    const datePart = [
+        date.getFullYear(),
+        pad(date.getMonth() + 1),
+        pad(date.getDate())
+    ].join("-");
+
+    const timePart = [
+        pad(date.getHours()),
+        pad(date.getMinutes()),
+        pad(date.getSeconds())
+    ].join("-");
+
+    return `${datePart}_${timePart}`;
+}