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.

backend/apps/api/src/auth/auth-session.controller.ts

@@ -0,0 +1,15 @@
+import { Body, Controller, Post } from '@nestjs/common';
+import { TokenExchangeService } from '../../../../libs/auth/src';
+import type { AuthorizationCodeExchangeRequest, AuthorizationCodeExchangeResult } from '../../../../libs/auth/src';
+
+@Controller('auth')
+export class AuthSessionController {
+  constructor(private readonly tokenExchange: TokenExchangeService) {}
+
+  @Post('session')
+  createSession(
+    @Body() dto: AuthorizationCodeExchangeRequest,
+  ): Promise<AuthorizationCodeExchangeResult> {
+    return this.tokenExchange.exchangeAuthorizationCode(dto);
+  }
+}