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.

api/src/lib.rs

@@ -168,6 +168,10 @@ pub struct Sender<'a> {
 #[derive(Default)]
 pub struct ServiceCtx {
     pub actions: Vec<NetAction>,
+    // Namespaced stat counters to bump (e.g. "nickserv.register"). The engine
+    // folds these into the shared registry after the command runs — so every
+    // service reports its own stats through one shared pipe.
+    pub stats: Vec<String>,
 }
 
 impl ServiceCtx {
@@ -179,6 +183,12 @@ impl ServiceCtx {
         });
     }
 
+    // Record a stat counter bump (namespaced, e.g. "chanserv.register"). Folded
+    // into the engine's shared registry, which the gRPC Stats API exposes.
+    pub fn count(&mut self, key: impl Into<String>) {
+        self.stats.push(key.into());
+    }
+
     // A channel/target message sourced from one of our pseudo-clients (e.g. a
     // bot answering a fantasy command, or BotServ SAY).
     pub fn privmsg(&mut self, from: &str, to: &str, text: impl Into<String>) {