Email owners before inactivity-expiry drops their account or channel

When [expire] warn_days is set and email is configured, the expiry sweep
now emails a heads-up to owners entering the warning window (inactive past
the threshold minus the lead time, but not yet expired) for whom an
address is on file — the founder's address for a channel. It's a chance to
keep the record by simply using it before the deadline.

Each idle spell warns at most once: a new AccountExpiryWarned /
ChannelExpiryWarned event marks the record, and the next activity stamp
clears the mark so a later idle spell warns afresh. The warning email is a
first-class api template (fedserv_api:📧:expiry_warning), alongside
the reset and confirm mails.

api/src/email.rs

@@ -69,6 +69,32 @@ pub fn confirm(brand: &str, accent: &str, logo: &str, account: &str, code: &str)
     }
 }
 
+// Warn the owner of an account or channel that inactivity will soon expire it.
+// `kind` is "account" or "channel", `remaining` a human span ("7 days"). The
+// remaining span takes the prominent code slot.
+pub fn expiry_warning(brand: &str, accent: &str, logo: &str, kind: &str, name: &str, remaining: &str) -> Mail {
+    let keep = if kind == "channel" {
+        "To keep it, have a member join the channel before then. Otherwise it will be removed."
+    } else {
+        "To keep it, just identify to it before then. Otherwise it will be removed."
+    };
+    Mail {
+        subject: format!("Your {kind} {name} is about to expire"),
+        text: format!(
+            "Your {kind} {name} has been inactive and will expire in {remaining}.\n{keep}\n"
+        ),
+        html: render(
+            brand,
+            accent,
+            logo,
+            "About to expire",
+            &format!("Your {kind} {name} has been inactive and will expire in {remaining}."),
+            remaining,
+            keep,
+        ),
+    }
+}
+
 // Escape the few characters that matter inside HTML text so a value can't break
 // out of the template.
 fn escape(s: &str) -> String {