Move backfill horizon from a startup env var into the editable profile

SMARTRUN_BACKFILL_HORIZON_DAYS was a server-startup-only env var with no UI,
defaulting to 3 years -- so editing the unrelated "Rolling window" profile
field (for classification, not sync) had no effect on how far back Sync Now
reached. Backfill horizon is now Profile.BackfillHorizonDays, read fresh on
every Backfill call, with its own field on the Profile page.

frontend/src/pages/Profile.tsx

@@ -118,6 +118,19 @@ export function Profile() {
         />
       </fieldset>
 
+      <fieldset className="kind-editor">
+        <legend>Sync</legend>
+        <NumberField
+          label="Backfill horizon (days)"
+          value={profile.BackfillHorizonDays}
+          onChange={(v) => set("BackfillHorizonDays", v)}
+        />
+        <p className="empty-state">
+          How far back "Sync now" reaches when walking backward from today. Not the same as the classification rolling
+          window above.
+        </p>
+      </fieldset>
+
       <fieldset className="kind-editor">
         <legend>Heart rate</legend>
         <NullableNumberField