A push notification model usually breaks in one of two ways: a second phone stops receiving anything, or notifications keep arriving for the account that just signed out. Both come from storing one push address per user. Store one row per app installation instead, with the user ID as a field, and someone with a phone and a tablet has two rows that a single query returns.

The sign-out half has an ordering trap. Delete the installation row while the user is still signed in, then call signOut. A security rule that compares the row owner to the signed-in user refuses the delete once there is no user left to compare against, and the device keeps pointing at the previous account.

The full write-up covers the Firestore rules, the Flutter registration listener, a server cleanup that handles both unregistered error codes, and an acceptance sequence to run on two test installations.

Great technical breakdown! Handling multi-device FCM tokens and secure sign-out correctly is essential for reliable push notifications. I appreciate the focus on both architecture and real-world edge cases. 👏