fix(insights): localise correlation narratives across all six locales

The generated-insight narrative layer built its user-facing sentences by
string interpolation and never routed through the i18n bundle, so a
non-English UI rendered English text spliced into localised copy — e.g.
"Higher activity steps tends to go with lower next-day weight" surfacing
mid-German on the Weight insight. Because those strings bypassed t(), the
locale-integrity guard could not catch them.

Route every correlation narrative through the server translator:

- Correlation discovery (daily lagged pairs + labs-vs-outcome) now takes
  the reader's locale, resolved from the correlations route the same way
  other insight generators resolve it. Sentence templates, the direction
  word, and every channel label live under a new insights.correlation.*
  namespace, translated in de/en/es/fr/it/pl. The three fixed hypotheses
  (BP-compliance, mood-pulse, weight-weekday) and their weekday labels are
  localised the same way; the analytics cache key now includes the locale
  so a language switch cannot serve another locale's cached text.
- Metric labels resolve to stable localised channel labels; the never-
  causal framing ("not a cause" / "an association, never a cause") and the
  faint/moderate/high hedging tiers are preserved in every language.

English output is byte-for-byte unchanged. Adds a regression test pinning
German narration and asserting it is not the English string.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018V8V9qapgHi7fhgtMJ8SkM

src/app/api/analytics/route.ts

@@ -22,6 +22,8 @@ import { resolveRestMode } from "@/lib/illness/rest-mode";
 import { buildHealthScoreBpInputs } from "@/lib/analytics/health-score-inputs";
 import { deriveBpWindow90 } from "@/lib/analytics/window-confidence";
 import { computeCorrelationHypothesesFastPath } from "@/lib/analytics/correlations-fast-path";
+import { resolveServerLocale } from "@/lib/i18n/server-locale";
+import { type Locale } from "@/lib/i18n/config";
 import {
   computeGlucoseClinicalMetrics,
   GLUCOSE_PANEL_WINDOW_DAYS,
@@ -96,12 +98,18 @@ export const GET = apiHandler(async (request?: Request) => {
   // thick body is the 30-query chain feeding the dashboard + insights
   // hero score; after a measurement sync the next mount serves the
   // prior body instantly while one background recompute refreshes it.
+  // v1.30.10 — the `correlations` block carries user-facing narration that is
+  // localised per reader, so the cache key MUST include the locale; otherwise a
+  // language switch would serve another locale's cached interpretation for up to
+  // the TTL. Every other block is locale-independent, so per-locale entries only
+  // cost a little duplication (most users have one locale).
+  const locale = await resolveServerLocale({ userLocale: user.locale ?? null });
   const cachedBody = await cachedSwr(
     caches.analytics as ServerCache<
       Awaited<ReturnType<typeof buildAnalyticsResponse>>
     >,
-    `${user.id}|default`,
-    () => buildAnalyticsResponse(user),
+    `${user.id}|default|${locale}`,
+    () => buildAnalyticsResponse(user, locale),
     annotate,
   );
 
@@ -173,7 +181,7 @@ function enrichLastSeenDaysAgo<
  * matches the v1.4.33 snapshot LRU's `buildCoachSnapshot` →
  * `buildCoachSnapshotImpl` shape.
  */
-async function buildAnalyticsResponse(user: AuthedUser) {
+async function buildAnalyticsResponse(user: AuthedUser, locale: Locale) {
   // v1.4.37.1 hotfix — fire-and-forget the rollup-fresh check.
   // Awaiting it on the read path can stall the Node event loop for
   // 30–60 s on a power-user account whose iOS step samples keep the
@@ -405,6 +413,7 @@ async function buildAnalyticsResponse(user: AuthedUser) {
     userTz,
     now: new Date(),
     coverage,
+    locale,
   });
 
   // v1.4.20 phase B5 — Personal Health Score. Server-deterministic

src/app/api/insights/correlations/route.ts

@@ -21,6 +21,7 @@ import { annotate } from "@/lib/logging/context";
 import { checkAnalyticsReadRateLimit } from "@/lib/rate-limit";
 import { requireAssistantSurface } from "@/lib/feature-flags";
 import { requireModuleEnabled } from "@/lib/modules/gate";
+import { resolveServerLocale } from "@/lib/i18n/server-locale";
 import { prisma } from "@/lib/db";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import type { MeasurementType } from "@/generated/prisma/client";
@@ -82,6 +83,11 @@ export const GET = apiHandler(async () => {
     select: { timezone: true },
   });
   const tz = profile?.timezone ?? "Europe/Berlin";
+  // Reader's locale for the narrated `interpretation` — the correlation cards
+  // render this string verbatim, so it MUST be localised (cookie / User.locale /
+  // Accept-Language). Without it the never-causal sentence leaked English into a
+  // non-English UI.
+  const locale = await resolveServerLocale({ userLocale: user.locale ?? null });
   const now = new Date();
   const since = new Date(now.getTime() - WINDOW_DAYS * MS_PER_DAY);
 
@@ -165,7 +171,7 @@ export const GET = apiHandler(async () => {
     series.push(envSeries);
   }
 
-  const result = discoverCorrelations(series);
+  const result = discoverCorrelations(series, { locale });
 
   // v1.22 — rolling early-detection pass over the trailing window, re-using
   // the already-built series (no extra DB read). Emerging pairs exclude anything
@@ -176,11 +182,14 @@ export const GET = apiHandler(async () => {
   );
   const emerging = discoverEmergingCorrelations(series, result, {
     recentFromDayKey,
+    locale,
   });
 
   // v1.22 — labs ↔ outcome pass (point-vs-window over sparse draws). Degrades
   // to absent when the user has too few draws to clear the per-pair floor.
-  const labCorrelations = discoverLabOutcomeCorrelations(labDraws, series);
+  const labCorrelations = discoverLabOutcomeCorrelations(labDraws, series, {
+    locale,
+  });
 
   annotate({
     action: { name: "insights.correlations.discover" },