Share pages: lazy, cached, our-own-words article summaries

The /a/<id> page now carries an original short summary so it stands on its own,
without republishing the publisher's article:
- summarize.py: transient SSRF-guarded fetch of the article text → local LLM
  writes a 2-4 sentence ORIGINAL summary (our words). Cached in article_summaries
  forever; we store only our summary, never the body. Generated lazily (only for
  shared/viewed articles), de-duped so concurrent hits don't double-generate.
- /a serves cached-or-pending; when pending it shows a calm "summary on its way,
  read at {source}" note and self-polls /api/summary/<id>, swapping the summary
  in the moment it's ready (never blocks the page on the batch-tier LLM).
- Share menu warms generation on open so recipients usually get the rich version.
- Container reaches the arbiter at arbiter:8080 over caddy_web (LLM env added to
  the API container). 124 tests pass.

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

frontend/src/lib/components/ArticleCard.svelte

@@ -57,6 +57,11 @@
     }
     shareOpen = false;
   }
+  // Kick off summary generation when the user signals intent to share, so it's
+  // cached by the time a recipient opens the link.
+  function warmSummary() {
+    fetch(`/api/summary/${article.id}`).catch(() => {});
+  }
   async function copy(text, label) {
     try {
       await navigator.clipboard.writeText(text);
@@ -127,7 +132,7 @@
         </button>
       {/if}
       <span class="sharewrap">
-        <button class="share" onclick={() => (shareOpen = !shareOpen)} aria-expanded={shareOpen}>Share</button>
+        <button class="share" onclick={() => { shareOpen = !shareOpen; if (shareOpen) warmSummary(); }} aria-expanded={shareOpen}>Share</button>
         {#if shareOpen}
           <button class="backdrop" aria-label="Close" onclick={() => (shareOpen = false)}></button>
           <div class="sharemenu" role="menu">