Prohibit OAuth client auth for revoked clients

Stash remote address in context

Encrypt outgoing emails

Sign outgoing emails

Print IP with "invalid source IP" authError

config: add GetOrigin function

This implements the same behavior as Python's srht.config.get_origin.

webhooks/legacy_test: verify headers are recorded

Deduplicate webhook headers on subsequent attempts

This also updates the database with the final payload headers, including
the signature and nonce, which are generated afresh for each attempt.

webhooks/legacy: increase specificity of logging

workers/legacy: correct Println with fmt directive

Make worker logs less loud

webhooks/legacy: address Postgres-specific issues

Update dowork

webhooks/legacy_test.go: style

webhooks/legacy: update only affected delivery

Forgot to add this where clause, whoops!

README.md: add build badge

Add legacy webhooks worker implementation

Expand context for work queues

server: use SO_REUSEADDR for main listener

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