modules: split services and the ircd link into external crates

nickserv, chanserv and the InspIRCd protocol move out of the binary into their
own workspace crates (fedserv-nickserv, fedserv-chanserv, fedserv-inspircd),
each depending only on fedserv-api. human_time and the branded account emails
move into the SDK crate so a module needs nothing from core; the engine keeps
its own inherent methods and builds emails via fedserv-api too. The bin now
constructs each module from its crate instead of an in-tree #[path] include.
Proves the SDK is self-sufficient: a third-party module is the same shape.

chanserv/src/topic.rs

@@ -0,0 +1,16 @@
+use fedserv_api::Store;
+use fedserv_api::{Sender, ServiceCtx};
+
+// TOPIC <#channel> <text>: set the channel topic (empty clears it).
+pub fn handle(me: &str, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, db: &dyn Store) {
+    let Some(&chan) = args.get(1) else {
+        ctx.notice(me, from.uid, "Syntax: TOPIC <#channel> <text>");
+        return;
+    };
+    if !super::require_op(me, from, chan, ctx, db) {
+        return;
+    }
+    let text = if args.len() > 2 { args[2..].join(" ") } else { String::new() };
+    ctx.topic(me, chan, &text);
+    ctx.notice(me, from.uid, format!("Topic for \x02{chan}\x02 updated."));
+}