database: expand field map for composite fields

This allows models to fetch multiple SQL columns to obtain the data
necessary to compute a single composite GraphQL field, while still
avoiding unnecessary fetches if those fields are not queried.

database.Apply: add support for non-pointer fields

database.WithTx: rollback if func returns error

Previously, this only rolled back if the called function panicked. This
updates it to also roll back if the called function returns an error.

Add legacy webhooks worker implementation

Expand context for work queues

Add basic auth tests

Import GQL server interfaces from gql.sr.ht