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.

modules/groupserv/src/lib.rs

@@ -8,7 +8,7 @@
 //! `lib.rs` holds the dispatcher and the two shared guards; each command lives
 //! in its own file.
 
-use fedserv_api::{NetView, Sender, Service, ServiceCtx, Store};
+use echo_api::{NetView, Sender, Service, ServiceCtx, Store};
 
 #[path = "register.rs"]
 mod register;
@@ -68,7 +68,7 @@ fn account<'a>(me: &str, from: &'a Sender, ctx: &mut ServiceCtx) -> Option<&'a s
 }
 
 // Whether `who` may manage `group` (founder, or holds the F/f flag).
-fn can_manage(group: &fedserv_api::GroupView, who: &str) -> bool {
+fn can_manage(group: &echo_api::GroupView, who: &str) -> bool {
     group.founder.eq_ignore_ascii_case(who)
         || group.members.iter().any(|m| m.account.eq_ignore_ascii_case(who) && (m.flags.contains('F') || m.flags.contains('f')))
 }