Shared stats registry + gRPC Stats API

A single namespaced counter map on the engine that every service reports
into: ServiceCtx::count(key) is folded in after each command, the engine
bumps its own events (botserv.kick/ban/warn/greet/trigger), and a
per-service command counter (nickserv.command, chanserv.command, …) is
tallied in dispatch. stats_snapshot() merges those with live gauges from
the store (accounts/channels/bots/opers total).

Exposed over gRPC as a new Stats service (Get -> map<string,uint64>),
authorized by the same bearer token as Directory/Accounts — so any
consumer (a dashboard, Django) reads every module's stats through one
pipe. New service is additive to the proto; existing RPCs untouched.

proto/fedserv.proto

@@ -138,3 +138,15 @@ message ForceLogoutReply { Status status = 1; uint32 sessions_cleared = 2; }
 
 message GroupNickRequest { string nick = 1; string account = 2; }
 message UngroupNickRequest { string nick = 1; }
+
+// Stats API: a flat, namespaced counter map any service contributes to
+// (e.g. "nickserv.command", "botserv.kick", "channels.total"). Read-only.
+service Stats {
+  rpc Get(StatsRequest) returns (StatsResponse);
+}
+
+message StatsRequest {}
+message StatsResponse {
+  map<string, uint64> counters = 1;
+  uint64 collected_at = 2; // unix seconds when this snapshot was taken
+}