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.

e2e/playwright.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig } from "@playwright/test";
+
+// Points at the throwaway stack run.sh brings up (see README) — never the
+// user's real self-hosted instance, so tests can freely create/delete data.
+const baseURL = process.env.AURUM_E2E_BASE_URL ?? "http://localhost:3100";
+
+export default defineConfig({
+  testDir: "./tests",
+  fullyParallel: false, // tests share one backend/DB and assert on exact totals — no isolation between them
+  workers: 1,
+  retries: 0,
+  reporter: [["list"]],
+  use: {
+    baseURL,
+    viewport: { width: 1440, height: 900 },
+    trace: "retain-on-failure",
+    screenshot: "only-on-failure",
+  },
+});