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/lib/openapi/routes/measurements.ts

@@ -292,6 +292,9 @@ const sleepDebtResource = z.object({
     .describe(
       "Nights still needed before the debt is asserted (0 when ready).",
     ),
+  source: measurementSourceEnum.describe(
+    "v1.25.0 — the active source the debt is resolved FROM, picked off the user's `sleepDebt` source ladder. `COMPUTED` is HealthLog's own rolling-balance estimate (the only producer today); a provider value would be a device-native debt. Clients explain the figure when it is `COMPUTED`.",
+  ),
 });
 
 const chronotypeBandEnum = z.enum([