Admin step A: privacy-respecting first-party event logging

- events table (kind, article_id, visitor_hash, day) with a UNIQUE key that dedups
  to one row per visitor-day — caps volume and makes counts mean distinct
  visitor-days. NO ip/ua/referrer/url. Groupings derived from article_id at query
  time, never stored.
- POST /api/events (public): whitelisted kinds (visit/open/share_ub/copy_source/
  native_share/source_click); visitor token hashed server-side (never raw).
- Frontend analytics.js: random localStorage visitor token; track() via sendBeacon;
  visit once/day; open on article click; share_ub/copy_source/native_share from the
  share menu; /a landing pages fire source_click. 127 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/ArticleCard.svelte

@@ -1,7 +1,13 @@
 <script>
   import { auth, savedIds, toggleSave } from '$lib/auth.svelte.js';
+  import { track } from '$lib/analytics.js';
 
   let { article, onaction, onreplace, ontag, onimageerror, onview, hero = false } = $props();
+
+  function opened() {
+    onview?.(article);
+    track('open', article.id);
+  }
   let isSaved = $derived(savedIds.has(article.id));
 
   const humanize = (s) => (s || '').replace(/-/g, ' ');
@@ -52,6 +58,7 @@
   async function nativeShare() {
     try {
       await navigator.share({ title: article.title, url: shareUrl });
+      track('native_share', article.id);
     } catch {
       /* user cancelled */
     }
@@ -80,7 +87,7 @@
   data-topic={article.topic ?? ''}
 >
   {#if showImage}
-    <a class="media" href={safeHref} target="_blank" rel="noopener" onclick={() => onview?.(article)}>
+    <a class="media" href={safeHref} target="_blank" rel="noopener" onclick={opened}>
       <img src={article.image_url} alt="" loading="lazy" referrerpolicy="no-referrer"
            onerror={() => { failed = true; onimageerror?.(); }} />
     </a>
@@ -101,7 +108,7 @@
     </div>
     <div class="src">{article.source}</div>
 
-    <h3><a href={safeHref} target="_blank" rel="noopener" onclick={() => onview?.(article)}>{article.title}</a></h3>
+    <h3><a href={safeHref} target="_blank" rel="noopener" onclick={opened}>{article.title}</a></h3>
 
     {#if hero && article.description}
       <p class="desc">{article.description}</p>
@@ -139,10 +146,10 @@
             {#if canNativeShare}
               <button role="menuitem" onclick={nativeShare}>Share…</button>
             {/if}
-            <button role="menuitem" onclick={() => copy(shareUrl, 'link')}>
+            <button role="menuitem" onclick={() => { copy(shareUrl, 'link'); track('share_ub', article.id); }}>
               {copied === 'link' ? 'Copied!' : 'Copy link'}
             </button>
-            <button role="menuitem" onclick={() => copy(article.url, 'source')}>
+            <button role="menuitem" onclick={() => { copy(article.url, 'source'); track('copy_source', article.id); }}>
               {copied === 'source' ? 'Copied!' : 'Copy source link'}
             </button>
           </div>