feat(insights): make sleep debt a configurable source ladder with a computed explainer

Generalise sleep debt onto the existing per-metric source-priority picker
(a new sleepDebt ladder, native-above-computed, default WHOOP > Oura > Polar >
computed) instead of assuming one provider. The rhythm read resolves the active
source off the ladder and carries it on the DTO; when the active source is our
own COMPUTED engine — every user today, since no provider ships a native
sleep-debt balance — the Sleep page explains what the figure means and that it
differs from a wearable's native number. The computed model itself is unchanged
and correct (a short rolling balance of need minus sleep, surplus-credited);
only stale comments that still referenced the old 14-night window are corrected.

src/components/settings/sources-section.tsx

@@ -89,6 +89,8 @@ const METRIC_LABEL_KEYS: Record<SourcePriorityMetricKey, string> = {
   recovery: "settings.sections.sources.metrics.recovery",
   // v1.18.10 I-5 — native-vs-derived stress ladder.
   stress: "settings.sections.sources.metrics.stress",
+  // v1.25.0 — native-vs-computed sleep-debt ladder.
+  sleepDebt: "settings.sections.sources.metrics.sleepDebt",
 };
 
 const SOURCE_LABEL_KEYS: Record<string, string> = {