fix(data): canonicalize locale pages

packages/stats/app/src/routes/stats-cache.ts

@@ -0,0 +1,27 @@
+const statsPageCacheControl = "public, max-age=60, s-maxage=300, stale-while-revalidate=86400"
+
+export function setStatsPageCacheHeaders(headers: Headers | undefined) {
+  if (!headers) return
+
+  headers.set("Cache-Control", statsPageCacheControl)
+  appendVary(headers, "Accept-Language", "Cookie")
+}
+
+function appendVary(headers: Headers, ...values: string[]) {
+  const existing = headers
+    .get("vary")
+    ?.split(",")
+    .map((value) => value.trim())
+    .filter(Boolean)
+
+  headers.set(
+    "vary",
+    values
+      .reduce(
+        (result, value) =>
+          result.some((item) => item.toLowerCase() === value.toLowerCase()) ? result : [...result, value],
+        existing ?? [],
+      )
+      .join(", "),
+  )
+}