Write the brand in lowercase where people see it

The name is inbuxa, lowercase, like the wordmark; INBUXA reads as an
acronym. The admin and webmail already changed. Here that's everything
the server shows people: the brand macro behind the protocol greetings,
the HTTP and SCIM realms, the startup banner and the calendar and contact
PRODID; the first-party OAuth client descriptions; the legacy-protocol
refusals; the default calendar and address book names and the SMTP
greeting default, in the code and the schema served to the admin
(checksum regenerated); startup and shutdown events; the User-Agent;
the sign-in and RSVP pages; the service units; the OpenAPI realm; the
crate descriptions and the README, where it's set in bold.

Identifiers that are uppercase for their own reasons stay: INBUXA_*
settings, SUBSPACE_INBUXA. So do code comments and the AGPL 5(a) notice
lines.

Tests follow: the IMAP ID name, the default collection names, the PRODID
in the iTIP fixtures and the CalDAV free-busy expectations, and the e2e
legacy-protocol refusals. The webdav, imap and jmap suites pass, so do
the unit tests of every crate touched, and 73 of 75 SMTP tests; of the
other two, antispam fails on main too, and queue_retry is a timing flake
that passes on its own.

crates/trc/src/event/enums_impl.rs

@@ -3729,8 +3729,8 @@ impl EventType {
             EventType::Security(SecurityEvent::LegacyProtocolsChanged) => {
                 "Legacy mail protocols switch changed"
             }
-            EventType::Server(ServerEvent::Startup) => "Starting INBUXA Server",
-            EventType::Server(ServerEvent::Shutdown) => "Shutting down INBUXA Server",
+            EventType::Server(ServerEvent::Startup) => "Starting inbuxa Server",
+            EventType::Server(ServerEvent::Shutdown) => "Shutting down inbuxa Server",
             EventType::Server(ServerEvent::StartupError) => "Server startup error",
             EventType::Server(ServerEvent::ThreadError) => "Server thread error",
             EventType::Server(ServerEvent::Licensing) => "Server licensing event",
@@ -3983,7 +3983,7 @@ impl EventType {
             EventType::Auth(AuthEvent::ClientRegistration) => "Authentication error",
             // inbuxa: legacy-protocols LP-6
             EventType::Auth(AuthEvent::LegacyProtocolRefused) => {
-                "This server allows only INBUXA webmail and JMAP apps"
+                "This server allows only inbuxa webmail and JMAP apps"
             }
             EventType::Auth(AuthEvent::Error) => "Authentication error",
             EventType::Auth(AuthEvent::CredentialExpired) => "Credential expired",