I need to bill users registered on Flus. This billing system doesn't have to be in the core of FreshRSS so it'll be developed in this extension. Note: I planned to use the "Subscription" term, but it's already used in FreshRSS :/

I'll stay with a basic workflow to start. I don't plan to have automatic renewal in a first time. Also, the payment system will come in a second time. This first version will basically just set up the renewal form and logic.

Billing will be available from the main menu, via a new "Billing" entry, under the "Extension" entry.

The page will show the date until when the subscription is valid and a button will offer to "renew" the subscription. A page showing how much it costs to renew appears, with a switch or a radio button to choose between monthly or annual renewal, and two "illustrated radio buttons" to choose between paying by card or by Paypal. A "Pay" button brings the user to the payment service. Two pages should be implemented: one for the successful payments, the other for the failures. The successful payment should show for how many month/year the user renewed, and the date until its subscription is valid.

As an administrator, I may want to let some users have no need to subscribe, it will be done manually in the configuration files, but the screens need to take care of this case. The button to renew should always be visible in case the user wants to do a special "support".

Raw illustration:

This PR doesn't have to implement the interface with the payment systems.

UX considerations:

• the "renew" button should become "primary" at subscription_end_at - 1.week
• the default choices (payment frequency and type) should be remembered from the last time
• it might be interesting to show for how long the subscription is valid on the main page

Technical considerations:

• a new hook has to be added in the extensions system to add entries in the menu
• at least 3 values need to be stored in the user (saving custom keys from extensions needs to be confirmed):
  • subscription_end_at (a date or null, set to now + 1.month at registration)
  • subscription_frequency (monthly or annually, monthly by default)
  • subscription_payment_type (card or paypal, card by default)
• the renewal "algo" must be subscription_end_at = max(subscription_end_at, now) + 1.month # or year
• a user can have a subscription_end_at set to null to have a free plan
• payments need to be saved somewhere but I'm not sure it's a good idea to have them in the user config.php file. Maybe in a separated db but… meh! I need to think more about it. -> delayed to a next ticket

Follow-up of my progress:

• Add a hook to add entries to the menu and to initialize configuration at the end of FreshRSS init (see FreshRSS/FreshRSS#2482)
• Add a "Billing" entry to the configuration menu
• Add support for custom default configuration from extensions (not necessary)
• Add a subscription_end_at item in the users configuration (default now + 1.month)
• Display the subscription end date on the billing main page
• Add the renewal form (the "Pay" button should act as if the payment is successful for the moment)
• Block user to ask him/her to renew its subscription
• (nice to have) Add support for no-billing users
• (nice to have) Configure the amounts of subscription
• (nice to have) Remember choices of payment frequency and type
• (nice to have) Transform "renew" button into a primary one 2 weeks before subscription_end_at