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/hooks/useTags.ts

@@ -0,0 +1,25 @@
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { createTag, deleteTag, fetchTags } from "@/api/tags";
+
+export function useTags() {
+  return useQuery({ queryKey: ["tags"], queryFn: fetchTags });
+}
+
+export function useCreateTag() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (name: string) => createTag(name),
+    onSuccess: () => queryClient.invalidateQueries({ queryKey: ["tags"] }),
+  });
+}
+
+export function useDeleteTag() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (id: number) => deleteTag(id),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ["tags"] });
+      queryClient.invalidateQueries({ queryKey: ["transactions"] });
+    },
+  });
+}