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>

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.category import Category
 from app.models.goal import Goal, GoalContribution
 from app.models.recurring import RecurringTransaction
 from app.models.settings import AppSettings
+from app.models.tag import Tag
 from app.models.transaction import Transaction
 
 __all__ = [
@@ -17,5 +18,6 @@ __all__ = [
     "Goal",
     "GoalContribution",
     "RecurringTransaction",
+    "Tag",
     "Transaction",
 ]