grpc: directory replication API for websites to mirror accounts/channels

Snapshot (full read) + Subscribe (live stream) over the same committed-entry
channel gossip uses. Identity and metadata only — password hashes, SCRAM
verifiers, cert fingerprints, and the finer channel-ops-list events never
cross this API. Bearer-token authenticated, optional server TLS. Verified
end-to-end against a live test-net registration, not just unit tests.

build.rs

@@ -0,0 +1,10 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Server only: the Rust side never needs to dial itself as a gRPC client.
+    // Django (or any other consumer) generates its own stub from proto/fedserv.proto.
+    tonic_build::configure()
+        .build_server(true)
+        .build_client(false)
+        .compile_protos(&["proto/fedserv.proto"], &["proto"])?;
+    println!("cargo:rerun-if-changed=proto/fedserv.proto");
+    Ok(())
+}