Re-add propagation chain for initial login

Device cookies handle subsequent auth changes (logout/re-login),
but the initial login needs the chain to create grants on each app
and link them to device cookies. Dead apps skipped via health check.

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

bp/auth/routes.py

@@ -271,8 +271,52 @@ def register(url_prefix="/auth"):
         # Fresh account session ID for grant tracking
         qsession[ACCOUNT_SESSION_KEY] = secrets.token_urlsafe(32)
 
+        # Propagate login to all client apps via OAuth chain
         redirect_url = pop_login_redirect_target()
-        return redirect(redirect_url, 303)
+        qsession["sso_final"] = redirect_url
+        qsession["sso_chain"] = list(ALLOWED_CLIENTS)
+        return redirect(url_for("auth.propagate"), 303)
+
+    @auth_bp.get("/propagate")
+    @auth_bp.get("/propagate/")
+    async def propagate():
+        """Chain through each client app's OAuth login to create grants
+        and set device cookies.  Dead apps are skipped."""
+        import os, aiohttp
+        from urllib.parse import quote
+
+        chain = qsession.get("sso_chain", [])
+        final = qsession.get("sso_final", account_url("/"))
+
+        if not g.get("user"):
+            qsession.pop("sso_chain", None)
+            qsession.pop("sso_final", None)
+            return redirect(final)
+
+        comeback = account_url("/auth/propagate")
+
+        while chain:
+            next_app = chain.pop(0)
+            qsession["sso_chain"] = chain
+
+            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():