feat(affiliate): use build_affiliate_url() in /go/<slug> redirect

Program-based products now get URLs assembled from the template at
redirect time. Changing a program's tracking_tag propagates instantly
to all its products without rebuilding. Legacy products (no program_id)
still use their baked affiliate_url via fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/padelnomics/app.py

@@ -291,7 +291,7 @@ def create_app() -> Quart:
         Uses 302 (not 301) so every hit is tracked — browsers don't cache 302s.
         Extracts article_slug and lang from Referer header best-effort.
         """
-        from .affiliate import get_product, log_click
+        from .affiliate import build_affiliate_url, get_product, log_click
         from .core import check_rate_limit
 
         # Extract lang from Referer path (e.g. /de/blog/... → "de"), default de
@@ -314,14 +314,17 @@ def create_app() -> Quart:
         if not product:
             abort(404)
 
+        # Assemble URL from program template; falls back to baked affiliate_url
+        url = build_affiliate_url(product, product.get("_program"))
+
         ip = request.remote_addr or "unknown"
         allowed, _info = await check_rate_limit(f"aff:{ip}", limit=60, window=60)
         if not allowed:
             # Still redirect even if rate-limited; just don't log the click
-            return redirect(product["affiliate_url"], 302)
+            return redirect(url, 302)
 
         await log_click(product["id"], ip, article_slug, referer or None)
-        return redirect(product["affiliate_url"], 302)
+        return redirect(url, 302)
 
     # Legacy 301 redirects — bookmarked/cached URLs before lang prefixes existed
     @app.route("/terms")