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.

tests/src/system/mod.rs

@@ -7,6 +7,7 @@
 pub mod antispam;
 pub mod authentication;
 pub mod authorization;
+pub mod branding;
 pub mod crypto;
 pub mod delivery;
 pub mod directory;