Skip dead apps in login propagation chain

Health-check each app via internal URL before redirecting.
Dead apps are silently skipped so the chain doesn't break.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/auth/routes.py

@@ -242,22 +242,44 @@ def register(url_prefix="/auth"):
         """Chain through each client app's OAuth login to propagate the
         account session.  Each app does its OAuth flow (instant since
         account is already logged in) then redirects back here.  When
-        the chain is empty, redirect to the original target."""
+        the chain is empty, redirect to the original target.
+        Dead apps are skipped via internal health check."""
+        import os, aiohttp
+        from urllib.parse import quote
+
         chain = qsession.get("sso_chain", [])
         final = qsession.get("sso_final", account_url("/"))
 
-        if not chain or not g.get("user"):
+        if not g.get("user"):
             qsession.pop("sso_chain", None)
             qsession.pop("sso_final", None)
             return redirect(final)
 
-        next_app = chain.pop(0)
-        qsession["sso_chain"] = chain
-
-        from urllib.parse import quote
         comeback = account_url("/auth/propagate")
-        login_url = app_url(next_app, f"/auth/login?next={quote(comeback, safe='')}")
-        return redirect(login_url)
+
+        while chain:
+            next_app = chain.pop(0)
+            qsession["sso_chain"] = chain
+
+            # Health check via internal URL before redirecting
+            internal = (os.getenv(f"INTERNAL_URL_{next_app.upper()}") or f"http://{next_app}:8000").rstrip("/")
+            try:
+                async with aiohttp.ClientSession() as http:
+                    async with http.head(
+                        internal,
+                        timeout=aiohttp.ClientTimeout(total=2),
+                        allow_redirects=True,
+                    ) as resp:
+                        if resp.status < 500:
+                            login = app_url(next_app, f"/auth/login?next={quote(comeback, safe='')}")
+                            return redirect(login)
+            except Exception:
+                current_app.logger.warning("[propagate] skipping dead app: %s", next_app)
+                continue
+
+        qsession.pop("sso_chain", None)
+        qsession.pop("sso_final", None)
+        return redirect(final)
 
     @auth_bp.post("/logout/")
     async def logout():