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/engine/state.rs

@@ -3,7 +3,7 @@ use std::time::{SystemTime, UNIX_EPOCH};
 
 // The read-only network view a module sees; re-exported so the engine keeps
 // naming it locally.
-pub use fedserv_api::{IncidentView, NetView, SeenView};
+pub use echo_api::{IncidentView, NetView, SeenView};
 
 // The most recent moderation/action incidents kept for LOGSEARCH.
 const INCIDENT_CAP: usize = 10_000;