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/run.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# Spins up a throwaway Aurum stack for E2E tests, runs Playwright against it,
+# then always tears the stack down again — including its Postgres volume.
+#
+# Fully isolated from the real self-hosted instance:
+#   - separate compose project name (aurum-e2e)      -> separate containers/network
+#   - separate Postgres database name (aurum_e2e)     -> separate data, same server
+#   - separate host port (3100, vs the real one's 3000) -> no port clash
+#   - `down -v` on exit removes the isolated volume, so no test data persists
+#     and the real `aurum`/`aurum_pgdata` are never opened.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+export AURUM_POSTGRES_DB=aurum_e2e
+export AURUM_POSTGRES_PASSWORD=aurum-e2e-only
+export AURUM_WEB_PORT=3100
+COMPOSE="docker compose -p aurum-e2e"
+
+cleanup() {
+  $COMPOSE down -v
+}
+trap cleanup EXIT
+
+$COMPOSE up -d --build
+
+echo "waiting for http://localhost:${AURUM_WEB_PORT} ..."
+for _ in $(seq 1 60); do
+  if curl -sf "http://localhost:${AURUM_WEB_PORT}" >/dev/null; then
+    echo "stack is up"
+    break
+  fi
+  sleep 1
+done
+
+cd e2e
+npm install
+AURUM_E2E_BASE_URL="http://localhost:${AURUM_WEB_PORT}" npx playwright test "$@"