fix(consent): close the receipt mint race, rate-limit, and share the back link

Mint the web AI-consent receipt inside a serializable transaction with an
in-transaction recheck, backed by a partial unique index over one active
receipt per user and kind, so two concurrent mounts can no longer leave two
active grants; a duplicate maps to a no-op mint. Supersede an active row
against the server clock rather than the client-supplied timestamp so a
backdated grant cannot invert the audit chain, and revoke through a single
atomic update. Rate-limit the consent endpoints per user. Extract the
hand-rolled sub-page back link into one component and adopt it across the
four pages that duplicated it.

src/components/ui/__tests__/back-link.test.tsx

@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { BackLink } from "../back-link";
+
+describe("<BackLink>", () => {
+  it("renders an anchor to the href carrying the label", () => {
+    const html = renderToStaticMarkup(
+      <BackLink href="/insights/workouts" label="Back to workouts" />,
+    );
+    expect(html).toContain('href="/insights/workouts"');
+    expect(html).toContain("Back to workouts");
+    // The decorative chevron is hidden from screen readers; the label
+    // carries the meaning.
+    expect(html).toContain('aria-hidden="true"');
+  });
+
+  it("keeps the shared left-bleed alignment on the control", () => {
+    const html = renderToStaticMarkup(
+      <BackLink href="/medications" label="Back" />,
+    );
+    // The `-ml-2 w-fit` left-bleed is the standard back-nav placement; it
+    // must survive on every adopting page.
+    expect(html).toContain("-ml-2");
+    expect(html).toContain("w-fit");
+  });
+
+  it("threads the optional data-slot for e2e/visual targeting", () => {
+    const html = renderToStaticMarkup(
+      <BackLink
+        href="/insights"
+        label="Back"
+        dataSlot="composite-score-back"
+      />,
+    );
+    expect(html).toContain('data-slot="composite-score-back"');
+  });
+});

src/components/ui/back-link.tsx

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+import Link from "next/link";
+import { ArrowLeft } from "lucide-react";
+
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+
+/**
+ * v1.16.16 — shared back-navigation control.
+ *
+ * The detail / sub-page surfaces (`/insights/workouts/[id]`,
+ * `/insights/values/[type]`, `/insights/scores/[metric]`,
+ * `/medications/[id]`) each hand-rolled the identical
+ * `Button(ghost, sm, -ml-2 w-fit) > Link > ArrowLeft + label` block. This
+ * folds that into one component so the back-nav reads the same — same hit
+ * area, same icon, same left-bleed alignment — on every page, and a future
+ * change to the affordance lives in one place.
+ *
+ * The label travels in from the caller (each page already owns its own
+ * translation key, e.g. `insights.workouts.detail.backToList`), so this
+ * component adds no new i18n surface.
+ */
+export interface BackLinkProps {
+  /** Internal href the back-nav points at. */
+  href: string;
+  /** Already-translated label text. */
+  label: string;
+  /** Optional per-surface `data-slot` for e2e/visual targeting. */
+  dataSlot?: string;
+  className?: string;
+}
+
+export function BackLink({ href, label, dataSlot, className }: BackLinkProps) {
+  return (
+    <Button
+      asChild
+      variant="ghost"
+      size="sm"
+      data-slot={dataSlot}
+      className={cn("-ml-2 w-fit", className)}
+    >
+      <Link href={href}>
+        <ArrowLeft className="size-4" aria-hidden="true" />
+        {label}
+      </Link>
+    </Button>
+  );
+}