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",
 ]

backend/app/models/tag.py

@@ -0,0 +1,26 @@
+"""A free-form, reusable label on transactions — orthogonal to Category:
+a category answers "what kind of spend is this", a tag answers "which
+event/project does it belong to" (e.g. "trip:georgia", "tax-deductible").
+A transaction can carry any number of tags, a tag can be reused across any
+number of transactions — see transaction_tags below.
+"""
+from sqlalchemy import Column, ForeignKey, String, Table
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base import Base
+
+transaction_tags = Table(
+    "transaction_tags",
+    Base.metadata,
+    Column("transaction_id", ForeignKey("transactions.id", ondelete="CASCADE"), primary_key=True),
+    Column("tag_id", ForeignKey("tags.id", ondelete="CASCADE"), primary_key=True),
+)
+
+
+class Tag(Base):
+    __tablename__ = "tags"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String(50), nullable=False, unique=True)
+
+    transactions: Mapped[list["Transaction"]] = relationship(secondary=transaction_tags, back_populates="tags")

backend/app/models/transaction.py

@@ -7,6 +7,7 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 from app.db.base import Base
 from app.models.enums import TransactionType
 from app.models.mixins import TimestampMixin
+from app.models.tag import transaction_tags
 
 
 class Transaction(Base, TimestampMixin):
@@ -33,3 +34,4 @@ class Transaction(Base, TimestampMixin):
     account: Mapped["Account"] = relationship(back_populates="transactions", foreign_keys=[account_id])
     transfer_account: Mapped["Account | None"] = relationship(foreign_keys=[transfer_account_id])
     category: Mapped["Category | None"] = relationship(back_populates="transactions")
+    tags: Mapped[list["Tag"]] = relationship(secondary=transaction_tags, back_populates="transactions")