Fix: CORS preflight 401'd by AuthMiddleware before CORSMiddleware (#3262)

AuthMiddleware is the outermost middleware, so a credential-less CORS preflight
(OPTIONS + Access-Control-Request-Method) was rejected with 401 before
CORSMiddleware could answer it. That blocks every cross-origin browser/WebView
client: the preflight fails, so the real request is never sent.

Let a genuine preflight through at the top of AuthMiddleware.dispatch via a pure,
unit-tested predicate (core.middleware.is_cors_preflight). Precise -- only
OPTIONS carrying Access-Control-Request-Method; a credentialed request is never
matched -- and no data access.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -54,7 +54,7 @@ from core.constants import (
     REQUEST_TIMEOUT, OPENAI_API_KEY,
 )
 from core.database import SessionLocal, ApiToken
-from core.middleware import SecurityHeadersMiddleware
+from core.middleware import SecurityHeadersMiddleware, is_cors_preflight
 from core.auth import AuthManager
 from core.exceptions import (
     SessionNotFoundError, InvalidFileUploadError,
@@ -253,6 +253,15 @@ if AUTH_ENABLED:
     class AuthMiddleware(BaseHTTPMiddleware):
         async def dispatch(self, request: Request, call_next):
             path = request.url.path
+            # A genuine CORS preflight (OPTIONS + Access-Control-Request-Method)
+            # carries no credentials by design and must reach CORSMiddleware to be
+            # answered. AuthMiddleware is the outermost middleware, so gating the
+            # preflight on auth 401s it before CORS can respond -- which blocks
+            # every cross-origin browser/WebView client before the real request
+            # is sent. Let real preflights through (only OPTIONS w/ the ACRM
+            # header; never a credentialed request).
+            if is_cors_preflight(request.method, request.headers):
+                return await call_next(request)
             if _is_auth_exempt(path):
                 return await call_next(request)
             # In-process internal-tool token bypass. Used by the agent