email.EnqueueStd: Don't overwrite headers

Sometimes we need to specify the Message-Id, From, and Reply-To headers
(e.g. for todo.sr.ht ticket notifications). Don't overwrite these
headers if they are present.

server: move auth middlware ahead of RealIP

So that @internal auth can correctly test against internal-ipnet.

Remove %e formatting verbs

%e is not valid for formatting error values:

> %e	scientific notation, e.g. -1.234456e+78

Instead, use %w when using fmt.Errorf (for error wrapping) and
%v when using log.Printf (%w is not valid in that context).

Enforce complexity limit on webhook payload

Configure server to manage mail queue

This also sets up the recovery function for webhooks, so we get emailed
when we panic during a webhook query.

server: add server reference to context

server: set max upload size to 1 GiB

Stash remote address in context

Add legacy webhooks worker implementation

Expand context for work queues

server: use SO_REUSEADDR for main listener

server: add /query/api-meta.json, docs

server: incorporate queue management

Simplify server module

Add email work queue

Import GQL server interfaces from gql.sr.ht