Branding and templates: per-domain, tenant and server logos, /logo, operator calendar email templates and RSVP page (BT-1 to BT-26)

Logos resolve domain, then tenant, then server-wide, then the built-in, with
subdomains finding their domain. GET /logo serves a data-URL image, redirects
to a URL logo without fetching it, sandboxes SVG, and answers 404 when no
custom logo applies. Emails embed the first PNG, JPEG or GIF logo. Logo and
template writes are checked; stored templates are read at send time, always
escaped, and fall back to the built-in with a build warning when they don't
parse. The RSVP page is served byte for byte with a CSP and no-referrer. The
sign-in and RSVP pages load the logo through an image element. MT-22's
session logo follows the chain to the server-wide logo.
Acceptance tests 1 to 17; test 18 written as the ignored branding_compat.

Cargo.lock

@@ -3326,6 +3326,7 @@ dependencies = [
  "http_proto",
  "hyper",
  "hyper-util",
+ "inbuxa-features",
  "jmap",
  "jmap_proto",
  "mail-auth",
@@ -3964,6 +3965,7 @@ name = "inbuxa-features"
 version = "0.16.22"
 dependencies = [
  "ahash",
+ "base64 0.23.1",
  "jmap_proto",
  "registry",
  "serde",