Rename the project to Echo (was fedserv)

Rebrand the daemon and workspace: the binary is now `echo`, every crate
is `echo-*` (`echo-api` and the module crates), Rust imports use
`echo_*`, the gRPC proto is `proto/echo.proto` with package `echo.v1`,
and the log filter, gossip peer-name default, and on-disk store default
(`echo.db.jsonl`) follow. Docs updated throughout. No behaviour change;
crate directories and the config schema are untouched.

src/link.rs

@@ -106,7 +106,7 @@ fn dispatch_email(email: &Option<crate::config::Email>, to: String, subject: Str
         let headers = format!("From: {}\r\nTo: {to}\r\nSubject: {subject}\r\nMIME-Version: 1.0\r\n", email.from);
         let msg = match html {
             Some(html) => {
-                let b = "fedserv-alt-boundary-x9";
+                let b = "echo-alt-boundary-x9";
                 format!(
                     "{headers}Content-Type: multipart/alternative; boundary=\"{b}\"\r\n\r\n\
                      --{b}\r\nContent-Type: text/plain; charset=utf-8\r\n\r\n{text}\r\n\