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/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;