Add Playwright e2e tests; fix two real bugs they caught

fix: OIDC_ISSUER_URL used Authentik's LAN IP (192.168.1.208:9443).
Authentik's discovery doc echoes back whichever host you query it
through, so that LAN IP got baked into authorization_endpoint -- the
URL the *browser* gets redirected to. Anyone off the LAN got sent to
an address they couldn't reach. Authentik was already publicly
exposed at auth.jerodrigged.com (pre-existing NPM proxy host); switched
to that, which also has a real cert so OIDC_ALLOW_INSECURE_TLS could
go back to false. Reported as "signed in via Authentik, redirected to
the local IP, failed."

fix: frontend's request() helper always sent Content-Type:
application/json, even for logout's bodyless POST. Fastify's default
JSON parser rejects an empty body under that content-type (400) --
sign-out silently failed to log the user out. curl-based testing
missed this because curl doesn't set that header without -d. Caught
immediately by the new Playwright local-login test.

e2e/: Playwright suite for local auth and OIDC login. OIDC test uses
a dedicated Authentik test user (blueprint-provisioned, never a real
personal login) so the whole flow can run unattended and repeatedly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

e2e/.env.example

@@ -0,0 +1,13 @@
+# Copy to .env, fill in, never commit the real one.
+
+BASE_URL=https://monitor.jerodrigged.com
+
+# Local auth (bcrypt) admin login.
+LOCAL_USERNAME=admin
+LOCAL_PASSWORD=
+
+# Dedicated Authentik test account (authentik_core.user blueprint on CT121,
+# path users/service-accounts, not a real person's login) — see
+# docs/oidc-setup.md. Never use a real personal Authentik password here.
+OIDC_USERNAME=playwright-test
+OIDC_PASSWORD=