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.

frontend/package.json

@@ -17,7 +17,6 @@
     "@angular/platform-browser": "^21.2.0",
     "@angular/router": "^21.2.0",
     "@angular/service-worker": "^21.2.0",
-    "oidc-client-ts": "^3.5.0",
     "rxjs": "~7.8.0",
     "tslib": "^2.3.0"
   },