fix(sources): ladder STRESS_SCORE so the source collapse stays deterministic

STRESS_SCORE has no source-priority ladder, so metricKeyForType returns null
and the SQL source collapse falls to an alphabetical rank-90 tiebreak. It is
safe today because the only producer is the COMPUTED HRV-derived proxy, but
the instant a device-native producer (Oura daily_stress) is enabled both rows
survive and blend nondeterministically.

Add a stress metric key with an OURA-above-COMPUTED default ladder, wire it
through metricKeyForType and RANKED_TYPES, and surface it in the Sources
settings (all six locales). A guard test asserts every RANKED_TYPES member
resolves to a non-null metric key so a future ranked type can't ship unladdered.
Regenerate the OpenAPI for the additive source-priority key.

src/components/settings/sources-section.tsx

@@ -87,6 +87,8 @@ const METRIC_LABEL_KEYS: Record<SourcePriorityMetricKey, string> = {
   skinTemperature: "settings.sections.sources.metrics.skinTemperature",
   respiratoryRate: "settings.sections.sources.metrics.respiratoryRate",
   recovery: "settings.sections.sources.metrics.recovery",
+  // v1.18.10 I-5 — native-vs-derived stress ladder.
+  stress: "settings.sections.sources.metrics.stress",
 };
 
 const SOURCE_LABEL_KEYS: Record<string, string> = {