feat(services): update convex service to include postgres

[aarsh21]

so apparently convex template was using sqlite which is by default but we can add postgres to convex. also to add postgres we need to have a particular name database which is self_hosted_convex.

• also added some other env varibles which were given in the convex docs

https://github.com/get-convex/convex-backend/tree/main/self-hosted#running-the-database-on-postgres

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also I don't know whether we should make another service for it.
For e.g: nextcloud with postgres

please let me know if we want that I will add a new template 😄

[Cinzya]

Yeah, it would be good if you could make this an entirely new service template instead of overriding the current one using sqlite, similar of how we do it with other services e.g. WordPress.
There might be people who would prefer using it with sqlite over postgres.

[peaklabs-dev]

Thank you for your contribution. Due to inactivity, we're closing this PR. If you'd like to revisit this change, feel free to reopen this PR or open a new pull request.