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/alembic/env.py

@@ -17,6 +17,7 @@ from app.models import (  # noqa: F401 — registers metadata
     Goal,
     GoalContribution,
     RecurringTransaction,
+    Tag,
     Transaction,
 )
 

backend/alembic/versions/f6a4d8b2e017_add_tags.py

@@ -0,0 +1,41 @@
+"""add tags and transaction_tags
+
+Revision ID: f6a4d8b2e017
+Revises: 8e2f5a13c9d4
+Create Date: 2026-08-25 11:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f6a4d8b2e017'
+down_revision: Union[str, None] = '8e2f5a13c9d4'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'tags',
+        sa.Column('id', sa.Integer(), nullable=False),
+        sa.Column('name', sa.String(length=50), nullable=False),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('name'),
+    )
+    op.create_table(
+        'transaction_tags',
+        sa.Column('transaction_id', sa.Integer(), nullable=False),
+        sa.Column('tag_id', sa.Integer(), nullable=False),
+        sa.ForeignKeyConstraint(['transaction_id'], ['transactions.id'], ondelete='CASCADE'),
+        sa.ForeignKeyConstraint(['tag_id'], ['tags.id'], ondelete='CASCADE'),
+        sa.PrimaryKeyConstraint('transaction_id', 'tag_id'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table('transaction_tags')
+    op.drop_table('tags')