feat: add free-form tags on transactions

Third of the four ideas picked from the Maybe/Sure feature audit. Tags are
orthogonal to categories (category = what kind of spend, tag = which
event/project) — many-to-many, created inline while typing in the
transaction form, filterable from the Transactions page.

Also adds tests/test_backup.py, the first test coverage the backup
export/import path has ever had, and fixes a MissingGreenlet bug it
surfaced in restore_backup's tag-wiring.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/transactions/TransactionsTable.tsx

@@ -53,6 +53,7 @@ export function TransactionsTable({ items, onEdit, onDelete, onJumpToMonth }: Tr
                   {formatTransactionDate(tx.date, Boolean(onJumpToMonth))} · {tx.account.name}
                   {isTransfer && tx.transfer_account_id ? ` ${t("transactions.transferSuffix")}` : ""}
                   {tx.category ? ` · ${translateCategoryName(tx.category.name)}` : ""}
+                  {tx.tags.length > 0 ? ` · ${tx.tags.map((tag) => tag.name).join(", ")}` : ""}
                 </span>
               </span>