stage-31 fix: home page 500 — replace bad jinja sum with precomputed count

Template had `{{ buckets|sum(attribute='items')|length }}` which tried
to sum lists starting at 0 → TypeError. Switched to a Python-side
`total_items = sum(len(b.items) for b in buckets)` passed through the
context and rendered directly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/psyc/cockpit/app.py

@@ -50,12 +50,14 @@ def index(request: Request) -> HTMLResponse:
             got = db.get_case(i.case_id)
             if not isinstance(got, Err):
                 case_index[i.case_id] = case_visuals.case_glyph_svg(got.value)
+    buckets = news_line.bucket_items(items)
     return TEMPLATES.TemplateResponse(
         request,
         "home.html",
         {
             "kpis": news_line.kpis(),
-            "buckets": news_line.bucket_items(items),
+            "buckets": buckets,
+            "total_items": sum(len(b.items) for b in buckets),
             "feeds": news_line.feed_health(),
             "featured": featured,
             "featured_hero": case_visuals.case_hero_svg(featured) if featured else "",

src/psyc/cockpit/templates/home.html

@@ -59,7 +59,7 @@
   <section class="panel home-news">
     <div class="panel-head">
       <h2>Recent activity</h2>
-      <span class="count">{{ buckets|sum(attribute='items')|length }} items</span>
+      <span class="count">{{ total_items }} items</span>
     </div>
     <p class="page-intro">Live feed of what psyc has detected and what it has done about it.</p>