Source feeds: click a source to see its publication feed

Click a source name on any card → a feed of just that source's articles,
newest-first, still accepted / non-duplicate / boundary-filtered (the calm
promise isn't bypassed). A natural way to follow a publication's feel.

* queries.feed + /api/feed: source_id filter; Article output gains source_id.
* Frontend: source label is a button → transient 'source:<id>' view (like
  'tag:<slug>'), rendered in the feed grid with Load more, header = source name.
* Ad-hoc, not a pinned lane. Foundation for a future source page (metadata) +
  Follow; shareable /source/<slug> route and source_view analytics come then.

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

frontend/src/lib/components/ArticleCard.svelte

@@ -2,7 +2,7 @@
   import { auth, savedIds, toggleSave } from '$lib/auth.svelte.js';
   import { track } from '$lib/analytics.js';
 
-  let { article, onaction, onreplace, ontag, onimageerror, onview, hero = false, thumb = false } = $props();
+  let { article, onaction, onreplace, ontag, onsource, onimageerror, onview, hero = false, thumb = false } = $props();
 
   function opened() {
     // Records history; the /a page itself fires the summary_viewed event on load.
@@ -122,7 +122,13 @@
         {/each}
       </div>
     </div>
-    <div class="src">{article.source}</div>
+    <div class="src">
+      {#if onsource && article.source_id}
+        <button type="button" class="srclink" onclick={() => onsource(article.source_id, article.source)}>{article.source}</button>
+      {:else}
+        {article.source}
+      {/if}
+    </div>
 
     <h3><a href={summaryHref} onclick={opened}>{article.title}</a></h3>
 
@@ -217,6 +223,11 @@
   .tile .pillrow { align-self: center; }
   /* Source on its own line below the tags, left-justified, for uniformity. */
   .src { color: var(--muted); font-size: 0.78rem; margin: -2px 0 2px; }
+  .srclink {
+    background: none; border: none; padding: 0; font: inherit; color: var(--muted);
+    cursor: pointer; border-bottom: 1px dotted transparent;
+  }
+  .srclink:hover { color: var(--accent-deep); border-bottom-color: var(--accent-soft); }
 
   h3 { font-size: 1.18rem; }
   h3 a:hover { color: var(--accent-deep); }