feat: switch oidc client to confidential (backend token exchange)

The provisioned IdP client (https://auth.forgecore.work) is confidential
rather than public/PKCE-only, so a client secret must never reach the
browser. The frontend now only performs the Authorization Code + PKCE
redirect itself (hand-rolled PKCE, oidc-client-ts dependency removed)
and hands the resulting code + verifier to a new, intentionally
unauthenticated POST /api/v1/auth/session endpoint, which performs the
code-for-tokens exchange server-side using OIDC_CLIENT_SECRET and
returns only {accessToken, expiresIn} — refresh_token/id_token are
never forwarded to the client.

New required backend env vars: OIDC_CLIENT_ID, OIDC_CLIENT_SECRET.
Added frontend/proxy.conf.json so the Angular dev server forwards
/api and /health to the local API without needing CORS.

.env.example

@@ -16,3 +16,7 @@ TLS_KEY_FILE=/etc/travel-planner/tls/tls.key
 OIDC_ISSUER=https://idp.example.invalid/realms/travel-planner
 OIDC_AUDIENCE=travel-planner-api
 OIDC_CLIENT_ID=travel-planner-web
+# OIDC_CLIENT_SECRET: this client is confidential (holds a secret). Never commit
+# a real value here; supply it only via the deployment host's secret store /
+# the developer's own shell environment.
+OIDC_CLIENT_SECRET=change-me-outside-source-control