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/api/tags.ts

@@ -0,0 +1,14 @@
+import { api } from "@/api/client";
+import type { Tag } from "@/types";
+
+export function fetchTags() {
+  return api.get<Tag[]>("/tags");
+}
+
+export function createTag(name: string) {
+  return api.post<Tag>("/tags", { name });
+}
+
+export function deleteTag(id: number) {
+  return api.delete<void>(`/tags/${id}`);
+}

frontend/src/api/transactions.ts

@@ -10,6 +10,7 @@ export interface TransactionFilters {
   end_date?: string;
   account_id?: number;
   category_id?: number;
+  tag_id?: number;
   type?: string;
   search?: string;
   sort?: TransactionSort;