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/lib/i18n.ts

@@ -138,6 +138,7 @@ const ru = {
   "transactions.income": "Доходы",
   "transactions.transfer": "Переводы",
   "transactions.allCategories": "Все категории",
+  "transactions.allTags": "Все теги",
   "transactions.failedToLoad": "Не удалось загрузить транзакции.",
   "transactions.noneFound": "Транзакций не найдено.",
   "transactions.searchPlaceholder": "Поиск по описанию, продавцу или заметке…",
@@ -167,6 +168,10 @@ const ru = {
   "transactions.form.noCategory": "Без категории",
   "transactions.form.merchantLabel": "Продавец (необязательно)",
   "transactions.form.notesLabel": "Заметка (необязательно)",
+  "transactions.form.tagsLabel": "Теги (необязательно)",
+  "transactions.form.tagsPlaceholder": "Начните вводить, чтобы найти или создать тег…",
+  "transactions.form.removeTag": "Убрать тег «{{name}}»",
+  "transactions.form.createTag": "Создать тег «{{name}}»",
   "transactions.viewNote": "Посмотреть заметку",
   "transactions.noteDialogTitle": "Заметка",
   "transactions.form.errorSelectAccount": "Выберите счёт",
@@ -501,6 +506,7 @@ const en: Record<keyof typeof ru, string> = {
   "transactions.income": "Income",
   "transactions.transfer": "Transfers",
   "transactions.allCategories": "All categories",
+  "transactions.allTags": "All tags",
   "transactions.failedToLoad": "Failed to load transactions.",
   "transactions.noneFound": "No transactions found.",
   "transactions.searchPlaceholder": "Search by description, merchant, or note…",
@@ -530,6 +536,10 @@ const en: Record<keyof typeof ru, string> = {
   "transactions.form.noCategory": "No category",
   "transactions.form.merchantLabel": "Merchant (optional)",
   "transactions.form.notesLabel": "Note (optional)",
+  "transactions.form.tagsLabel": "Tags (optional)",
+  "transactions.form.tagsPlaceholder": "Type to find or create a tag…",
+  "transactions.form.removeTag": "Remove tag \"{{name}}\"",
+  "transactions.form.createTag": "Create tag \"{{name}}\"",
   "transactions.viewNote": "View note",
   "transactions.noteDialogTitle": "Note",
   "transactions.form.errorSelectAccount": "Select an account",