feat(webhook): add support for dynamic placeholders in webhook URL

[0xSysR3ll]

Description

This PR introduces support for dynamic placeholders in webhook URLs, starting with {{requestedBy_username}}.
Users can enable this feature in the notification settings, and available placeholders are displayed as a list.

• Should we rename "placeholders" to "variables" or "tokens" for better clarity?
• Is the UI placement of the placeholders list intuitive, or should it be adjusted?
• Should we extend support for additional placeholders in future updates?

N.B: Marked this feature as experimental to gather feedback before finalization.

Screenshot (if UI-related)

To-Dos

• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required)

Issues Fixed or Closed

• Fixes Support for dynamic notification URLs #1472

[github-actions]

This pull request has merge conflicts. Please resolve the conflicts so the PR can be successfully reviewed and merged.

[github-actions]

This pull request has merge conflicts. Please resolve the conflicts so the PR can be successfully reviewed and merged.

[gauthier-th]

Should we rename "placeholders" to "variables" or "tokens" for better clarity?

IMO "variables" is more clear, but "placeholders" is fine too.

Is the UI placement of the placeholders list intuitive, or should it be adjusted? (Open to feedback!)

It may quickly be a bit messy if the list is too long, don't you think so?
We could just add a link to the documentation, like here: https://docs.jellyseerr.dev/using-jellyseerr/notifications/webhook#template-variables ?

Should we extend support for additional placeholders in future updates?

What's wrong with using the same placeholder system as current one in the webhook content itself?

[0xSysR3ll]

IMO "variables" is more clear, but "placeholders" is fine too.

Yep, totally makes sense !

It may quickly be a bit messy if the list is too long, don't you think so? We could just add a link to the documentation, like here: https://docs.jellyseerr.dev/using-jellyseerr/notifications/webhook#template-variables ?

As there was only one variable available, it wasn't necessary. But since we include them all, then yes I will do this.

What's wrong with using the same placeholder system as current one in the webhook content itself?

I did not think about making all variables available at first, but I guess you're right, it could be interesting for some users.