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.

crates/features/src/branding/mod.rs

@@ -0,0 +1,13 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! Branding and templates (`docs/spec/features/branding-and-templates.md`):
+//! logos per domain, tenant and server, and the operator's calendar email
+//! templates and RSVP page.
+
+pub mod logo;
+pub mod templates;
+pub mod writes;