test: real backend pytest + E2E Playwright suites, both isolated from prod data

backend/tests/: 33 pytest tests against a real Postgres database
(aurum_test, created/migrated/dropped per run) exercised through the actual
HTTP API — not sqlite, not mocks, and the real `aurum` database is never
opened (app lifespan is skipped, get_session is overridden per test).
Covers transaction category-kind validation, year/month/date-range
filtering and pagination, dashboard summary arithmetic, budget status, cash
flow, and the /transactions/years picker.

e2e/: 6 Playwright specs driving a real browser against a fully separate
throwaway docker-compose stack (run.sh: separate project name, database,
port, and volume, torn down with `down -v` on exit) — regression coverage
for the Recent Transactions month bug, the Card grid-overflow bug, the
frozen-year bug, the Net Worth default range, and the Dashboard->
Transactions deep link, all fixed earlier this session.

Also synced the backend's APP_VERSION constant (stale at 0.15.2, embedded
in exported backups) with the frontend version.

.gitignore

@@ -28,12 +28,18 @@ __pycache__/
 *.pyc
 .venv/
 *.egg-info/
+.pytest_cache/
 
 # Node / frontend
 node_modules/
 dist/
 .vite/
 
+# Playwright (e2e/) run artifacts
+/e2e/test-results/
+/e2e/playwright-report/
+/e2e/.playwright/
+
 # Editors / OS / local tooling
 .DS_Store
 .idea/