fix: add lastmod dates to sitemap for better Google indexing

Populate lastmod from actual data (diff dates, timeline enforcement
dates) so Google can prioritize crawling. Addresses 14 pages showing
as "not indexed" in Search Console.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wharfe

frontend/app/sitemap.ts

@@ -1,5 +1,10 @@
 import type { MetadataRoute } from "next";
-import { getDiffIds, getTimelineIds } from "@/lib/data";
+import {
+  getDiffIds,
+  getDiffData,
+  getTimelineIds,
+  getTimelineData,
+} from "@/lib/data";
 import { LIFE_THEMES } from "@/lib/life-themes";
 
 export const dynamic = "force-static";
@@ -11,28 +16,55 @@ export default function sitemap(): MetadataRoute.Sitemap {
   const lawIds = getTimelineIds();
   const themeIds = LIFE_THEMES.map((t) => t.id);
 
+  // Compute the most recent diff date for the home page lastmod
+  const allDiffDates = diffIds.map((id) => getDiffData(id).date_after);
+  const latestDiffDate =
+    allDiffDates.length > 0
+      ? allDiffDates.sort().reverse()[0]
+      : new Date().toISOString().slice(0, 10);
+
   return [
     // Static pages
-    { url: BASE_URL, changeFrequency: "weekly", priority: 1.0 },
-    { url: `${BASE_URL}/about`, changeFrequency: "monthly", priority: 0.5 },
+    {
+      url: BASE_URL,
+      lastModified: latestDiffDate,
+      changeFrequency: "weekly",
+      priority: 1.0,
+    },
+    {
+      url: `${BASE_URL}/about`,
+      changeFrequency: "monthly",
+      priority: 0.5,
+    },
 
     // Law pages (from timeline data)
-    ...lawIds.map((id) => ({
-      url: `${BASE_URL}/law/${id}`,
-      changeFrequency: "weekly" as const,
-      priority: 0.8,
-    })),
+    ...lawIds.map((id) => {
+      const data = getTimelineData(id);
+      // Timeline is sorted newest-first; use the first entry as lastmod
+      const latestEntry = data.timeline[0];
+      return {
+        url: `${BASE_URL}/law/${id}`,
+        lastModified: latestEntry?.enforcement_date,
+        changeFrequency: "weekly" as const,
+        priority: 0.8,
+      };
+    }),
 
     // Diff pages (from diff data files)
-    ...diffIds.map((id) => ({
-      url: `${BASE_URL}/diff/${id}`,
-      changeFrequency: "monthly" as const,
-      priority: 0.7,
-    })),
+    ...diffIds.map((id) => {
+      const data = getDiffData(id);
+      return {
+        url: `${BASE_URL}/diff/${id}`,
+        lastModified: data.date_after,
+        changeFrequency: "monthly" as const,
+        priority: 0.7,
+      };
+    }),
 
     // Theme pages (from life-themes config)
     ...themeIds.map((id) => ({
       url: `${BASE_URL}/theme/${id}`,
+      lastModified: latestDiffDate,
       changeFrequency: "weekly" as const,
       priority: 0.6,
     })),