fix(daily): decide the line under the Today lead on the server

The muted line under the lead repeated what the lead had just said: a lead
about blood pressure sat above "Your latest blood pressure is sitting in
the optimal band · ↓ ~10 mmHg…". The web client only dropped the headline
when the lead quoted it word for word, and other clients had no rule at
all. The digest now carries `signalLine`, built next to the lead: when the
lead already talks about the top signal's metric (matched in every shipped
language) or says its headline, only the delta remains, and with no delta
there is no line. The hero renders the field verbatim.

src/app/api/daily/digest/__tests__/route.test.ts

@@ -70,6 +70,7 @@ const DIGEST: DailyDigest = {
     composition: ["BLOOD_PRESSURE", "ACTIVITY", "SLEEP"],
   },
   topSignal: null,
+  signalLine: null,
   briefingLead: "Blood pressure is holding steady.",
   lead: { text: "Blood pressure is holding steady.", source: "briefing" },
   today: [],