Vishnu Mohandas e798ac02c6 [server] Check in with paid customers (#6106) ...

## Description
Update the `subscriptions` table to store the time at which
subscriptions were upgraded (`upgraded_at`).

Reach out to customers who upgraded 7 days ago to make sure all is well.
<img width="628" alt="Screenshot 2025-05-31 at 1 41 09 PM"
src="https://github.com/user-attachments/assets/7d1e970a-c7fa-4666-8d4f-db13ba7c105d"
/>

Store this information within `notification_history` so they are not
contacted again (in case of admin interventions).

> Note: We will not be back-filling data for existing subscriptions.

## Tests
- [x] Tested locally to make sure only customers who upgraded 7 days ago
were pinged.

2025-06-02 15:25:23 +05:30

..

main.go

[server] Check in with paid customers (#6106)

2025-06-02 15:25:23 +05:30