fix(auth): address PR #758 auto-review (id-token verify, nonce, CI key)

Blocking:
- AS proxy callback now calls verify_id_token before caching the proxy
  code so a tampered IdP response can't smuggle identity claims.

Important:
- Browser OAuth flow generates and verifies an OIDC nonce; new alembic
  migration 006 adds the nonce column to oauth_sessions.
- _origin_matches_self logs a warning when CSRF check is bypassed.
- oauth_tools.py uses get_shared_storage instead of fresh handles.

Nits:
- New token_utils.get_oidc_discovery shares the 5-minute cache with
  verify_id_token; oauth_login (integrated) and _revoke_refresh_token_at_idp
  now use it instead of issuing fresh discovery fetches.
- Drop typing.Optional from oauth_tools.py in favour of X | None.

CI:
- test.yml generates an ephemeral Fernet TOKEN_ENCRYPTION_KEY per run
  with openssl, removing the dependency on a missing repo secret.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/oauth_routes.py

@@ -954,6 +954,28 @@ async def _oauth_callback_as_proxy(
         f"(token_type={nc_token_response.get('token_type')})"
     )
 
+    # Verify the ID token signature + claims before caching the response
+    # (PR #758 finding 1). Without this, a compromised IdP or tampered
+    # transport could plant arbitrary identity claims into the proxy code
+    # entry that gets handed back to the MCP client. Mirrors the
+    # verification done in oauth_callback_nextcloud.
+    id_token = nc_token_response.get("id_token")
+    try:
+        await verify_id_token(
+            id_token,
+            discovery_url=discovery_url,
+            expected_audience=mcp_server_client_id,
+        )
+    except IdTokenVerificationError as e:
+        logger.error("AS proxy: ID token verification failed: %s", e)
+        return JSONResponse(
+            {
+                "error": "invalid_token",
+                "error_description": "ID token failed verification",
+            },
+            status_code=400,
+        )
+
     # Generate a proxy authorization code for the client
     proxy_code = secrets.token_urlsafe(32)
     _proxy_codes[proxy_code] = ProxyCodeEntry(