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.spec.ts

@@ -0,0 +1,23 @@
+import { AuthSessionController } from './auth-session.controller';
+
+describe('AuthSessionController', () => {
+  it('POST /auth/session exchanges the authorization code via the token exchange service', async () => {
+    const tokenExchange = {
+      exchangeAuthorizationCode: jest
+        .fn()
+        .mockResolvedValue({ accessToken: 'at-1', expiresIn: 3600 }),
+    };
+    const controller = new AuthSessionController(tokenExchange as never);
+
+    const dto = {
+      code: 'code-1',
+      codeVerifier: 'verifier-1',
+      redirectUri: 'http://localhost:4200/auth/callback',
+    };
+    await expect(controller.createSession(dto)).resolves.toEqual({
+      accessToken: 'at-1',
+      expiresIn: 3600,
+    });
+    expect(tokenExchange.exchangeAuthorizationCode).toHaveBeenCalledWith(dto);
+  });
+});