Support for DB_URI in mongodb database driver

Peekaping currently builds the mongodb URI from user-supplied variables (see [related code](https://github.com/0xfurai/peekaping/blob/d22eba52a5250aaa914a8df87bddd7478c6af449/apps/server/internal/infra/mongo.go#L48)). The main issue is that mongodb supports a lot of connection customization in URI form, as you can see in the [mongodb documentation](https://www.mongodb.com/docs/manual/reference/connection-string-self-managed-examples/#std-label-connections-connection-examples-self-managed)

In my specific case, I host a managed, shared mongodb cluster in replica-set style, and service discovery is via consul SRV. If I can supply my own URI, then I could simply use "mongodb+srv://" and the mongodb driver will automatically resolve the endpoints for peekaping

I think the user should be able to provide its own URI via an environment variable, and it should take precedence over the "unmounted" database variables. The [mongo-go-driver](https://github.com/mongodb/mongo-go-driver) automatically resolves the URI by itself but I'm not sure about [bun](https://github.com/uptrace/bun) for other database drivers

Also, I don't think hardcoding `authSource=admin` is a good practice

The code to modify is in apps/server/internal/infra/mongo.go around line 48. First, read the existing URI-building logic and the mongo-go-driver documentation. Add support for a DB_URI environment variable that overrides the constructed URI. Ensure the change works with bun for other databases. Test by setting the environment variable and verifying the connection works with a MongoDB SRV URI.

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