treewide: update to new multi worker dowork api

The work.Queue() implementation has changed and now allows scheduling
tasks from multiple goroutines in parallel. Also, it requires a new
argument for limiting the queue buffer size in order to apply back
pressure.

Define new configuration variables to allow tuning the queue sizes and
number of workers per service:

  [mail]
  # Maximum size of the outgoing email queue (default 512).
  egress-queue-size = 512

  [$service_name]
  # Number of parallel workers per queue (default 1).
  # There are multiple queues (for egress email, webhooks, etc.).
  # This setting is applied on a per-queue basis.
  queue-workers = 1

  [webhooks]
  # Maximum size of the webhooks queue (default 512).
  queue-size = 512

Use these new settings to configure the queues and workers accordingly.

Fix unit tests as well.

Link: https://git.sr.ht/~sircmpwn/dowork/commit/95719cfc0118
Signed-off-by: Robin Jarry <robin@jarry.cc>

webhooks: fix failing test

go fmt

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.

webhooks/legacy_test: verify headers are recorded

webhooks/legacy: address Postgres-specific issues

webhooks/legacy_test.go: style

webhooks/legacy: update only affected delivery

Forgot to add this where clause, whoops!

Add legacy webhooks worker implementation