Web vitals: prefer external RUM

lib/server/webVitals/mode.ts

@@ -0,0 +1,31 @@
+/**
+ * Web vitals persistence mode (CR-80). Default: external in production, local in dev.
+ * Does not require a specific observability vendor — ops can pipe structured logs to any stack.
+ */
+export type WebVitalsStorageMode = "external" | "local";
+
+const VALID: WebVitalsStorageMode[] = ["external", "local"];
+
+function normalizeEnv(
+  raw: string | undefined,
+): WebVitalsStorageMode | undefined {
+  const v = raw?.trim().toLowerCase();
+  if (v === "external" || v === "local") return v;
+  if (v === "database") {
+    // Reserved for Ticket 9 option C — not implemented yet; safe default.
+    return "external";
+  }
+  return undefined;
+}
+
+/**
+ * Resolves storage mode from `WEB_VITALS_STORAGE` or defaults:
+ * - `production` → `external` (no `.next` writes; ingest logs only)
+ * - otherwise → `local` (file-based under `.next/web-vitals` for dev/admin)
+ */
+export function getWebVitalsStorageMode(): WebVitalsStorageMode {
+  const fromEnv = normalizeEnv(process.env.WEB_VITALS_STORAGE);
+  if (fromEnv && VALID.includes(fromEnv)) return fromEnv;
+
+  return process.env.NODE_ENV === "production" ? "external" : "local";
+}