redis: use UniveralClient, support sentinel mode

This commit switches the redis client to using a UniversalClient [1],
which can operate in standard, cluster, or failover (sentinel) mode.

It also implements basic support for having (multiple)
`redis+sentinel://` URLs as connection URL in the config (e.g. for
`redis-host`).

This is the Go equivalent of the proposed Python patch [2]. It supports
the same examples given there.

[1] https://pkg.go.dev/github.com/go-redis/redis/v8#UniversalClient
[2] https://lists.sr.ht/~sircmpwn/sr.ht-dev/patches/55521

server: add AnonRouter function

To expose anonymous (unauthenticated) routing to downstream core-go
users.

server: fix playground on config.Debug

A recent change made it so that the playground required authentication
to access in debug mode; this moves it under the other auth-free
endpoints.

server: use routing groups

Instead of hardcoding some exceptions in the auth middleware, use a
different routing group for routes that do not require auth. Makes the
auth middleware more generic and also removes a lot of unneccessary
middleware processing from routes that don't need it.

For now, the added group is not accessible from outside the module, but
if the need arises, this might be an option.

Migrate to chi v5

v1 is not maintained anymore.

server: register SQL collector

Missed this step

Add prometheus SQL connection stats

Also updates the prometheus client_go library

Normalize remote address before saving to context

One cause for https://todo.sr.ht/~sircmpwn/meta.sr.ht/193 was that code
performing this sanitization was in multiple places, and a fix was not
applied in all places. There is no reasonable expectation for the port
to be present anyways, e.g. if the address is taken from a trusted
reverse proxy's header. Hence, perform the normalization here, so that
the code doing this in applications can be simplified.

Note that this does not yet fix the below ticket, it will just make the
fix easier.

References: https://todo.sr.ht/~sircmpwn/meta.sr.ht/193

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