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.

backend/tests/helpers.py

@@ -0,0 +1,22 @@
+"""Shared test fixtures' plain-Python helpers (not pytest fixtures
+themselves — those live in conftest.py)."""
+from decimal import Decimal
+
+
+def money(value) -> Decimal:
+    """Compares amounts by value regardless of whether the API serializes
+    Decimal as a JSON string or number — that's a wire-format detail, not
+    business behavior worth pinning a test to."""
+    return Decimal(str(value))
+
+
+def txn_payload(account_id: int, **overrides) -> dict:
+    payload = {
+        "account_id": account_id,
+        "type": "expense",
+        "amount": "10.00",
+        "description": "test transaction",
+        "date": "2026-01-15",
+    }
+    payload.update(overrides)
+    return payload