SEO flywheel: /today digest, sitemap, robots, home OG tags

Make the summary pages discoverable so traffic compounds passively:
- /today: a server-rendered, shareable + indexable digest of today's brief —
  each item's title (→ /a summary), our summary, and a source link. OG/Twitter
  meta + self-canonical.
- /sitemap.xml: dynamic — home, /today, and every accepted non-duplicate /a page
  with lastmod. robots.txt allows all and points to it.
- Home (SPA shell) gains canonical + OG/Twitter tags for cleaner unfurls.
- Caddy routes /today + /sitemap.xml to the API. 133 tests pass.

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

goodnews/api.py

@@ -800,6 +800,40 @@ def create_app() -> FastAPI:
         _kick_summary(article_id, background_tasks)
         return {"status": "pending", "summary": None}
 
+    @app.get("/today", response_class=HTMLResponse)
+    def today_digest() -> HTMLResponse:
+        with get_conn() as conn:
+            b = queries.brief(conn)
+        items = b.get("items") or []
+        if not items:
+            return HTMLResponse(share.render_not_found(PUBLIC_BASE_URL), status_code=404)
+        return HTMLResponse(share.render_digest(items, PUBLIC_BASE_URL, b.get("brief_date")))
+
+    @app.get("/sitemap.xml")
+    def sitemap() -> Response:
+        with get_conn() as conn:
+            rows = conn.execute(
+                "SELECT a.id, COALESCE(a.published_at, a.discovered_at) AS lm "
+                "FROM articles a JOIN article_scores s ON s.article_id = a.id "
+                "WHERE s.accepted = 1 AND a.duplicate_of IS NULL "
+                "ORDER BY lm DESC LIMIT 5000"
+            ).fetchall()
+        base = PUBLIC_BASE_URL
+        urls = [
+            f"<url><loc>{base}/</loc><changefreq>hourly</changefreq><priority>1.0</priority></url>",
+            f"<url><loc>{base}/today</loc><changefreq>daily</changefreq><priority>0.9</priority></url>",
+        ]
+        for r in rows:
+            lm = (r["lm"] or "")[:10]
+            lastmod = f"<lastmod>{lm}</lastmod>" if lm else ""
+            urls.append(f"<url><loc>{base}/a/{r['id']}</loc>{lastmod}</url>")
+        xml = (
+            '<?xml version="1.0" encoding="UTF-8"?>'
+            '<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">'
+            + "".join(urls) + "</urlset>"
+        )
+        return Response(content=xml, media_type="application/xml")
+
     @app.post("/api/import")
     def import_local(body: ImportBody, request: Request) -> dict:
         """Fold this device's anonymous history/saved into the account (one-time)."""