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/main.py

@@ -18,6 +18,7 @@ from app.api.routes import (
     recurring,
     reports,
     settings as settings_routes,
+    tags,
     transactions,
 )
 from app.core.config import APP_VERSION, get_settings
@@ -61,6 +62,7 @@ app.include_router(advice.router, prefix="/api")
 app.include_router(goals.router, prefix="/api")
 app.include_router(recurring.router, prefix="/api")
 app.include_router(cash_flow.router, prefix="/api")
+app.include_router(tags.router, prefix="/api")
 
 
 @app.get("/api/health")