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.

src/config.rs

@@ -43,6 +43,9 @@ pub struct Expire {
     pub accounts_days: u64,
     #[serde(default)]
     pub channels_days: u64,
+    // Days before expiry to email a warning to the owner (0 = no warning email).
+    #[serde(default)]
+    pub warn_days: u64,
 }
 
 impl Expire {
@@ -53,6 +56,10 @@ impl Expire {
     pub fn channel_ttl(&self) -> Option<u64> {
         (self.channels_days > 0).then(|| self.channels_days * 86_400)
     }
+    // The warning lead time in seconds, or None if warnings are off.
+    pub fn warn_ttl(&self) -> Option<u64> {
+        (self.warn_days > 0).then(|| self.warn_days * 86_400)
+    }
 }
 
 // The staff audit feed: notable service actions are announced to this channel