Set blog_did = account_did — one device identity across all apps

Callback adopts account's device_id by overwriting g.device_id,
so the factory after_request sets {app}_did cookie to account's value.
Simplifies factory check: g.device_id IS the account_did, no need
to read _account_did from session separately.

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

infrastructure/oauth.py

@@ -61,10 +61,13 @@ def create_oauth_blueprint(app_name: str) -> Blueprint:
     @bp.get("/callback")
     @bp.get("/callback/")
     async def callback():
-        # Always store account_did when account passes it back
+        # Adopt account's device id as our own — one identity across all apps
         account_did = request.args.get("account_did", "")
         if account_did:
             qsession["_account_did"] = account_did
+            # Overwrite this app's device cookie with account's device id
+            g.device_id = account_did
+            g._new_device_id = True  # factory after_request will set the cookie
 
         # Handle prompt=none error (user not logged in on account)
         error = request.args.get("error")