add simple A/B testing with @ab_test decorator and Umami data-tag

Assigns visitors to experiment variants via cookie (30-day), sets g.ab_variant
and g.ab_tag, and conditionally adds data-tag to the Umami script tag so all
pageviews and events are automatically tagged in the Umami dashboard.

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

padelnomics/src/padelnomics/app.py

@@ -92,6 +92,8 @@ def create_app() -> Quart:
             "is_admin": "admin" in (g.get("user") or {}).get("roles", []),
             "now": datetime.utcnow(),
             "csrf_token": get_csrf_token,
+            "ab_variant": getattr(g, "ab_variant", None),
+            "ab_tag": getattr(g, "ab_tag", None),
         }
 
     # Health check

padelnomics/src/padelnomics/core.py

@@ -4,6 +4,7 @@ Core infrastructure: database, config, email, and shared utilities.
 import hashlib
 import hmac
 import os
+import random
 import re
 import secrets
 import unicodedata
@@ -15,7 +16,7 @@ from pathlib import Path
 import aiosqlite
 import resend
 from dotenv import load_dotenv
-from quart import g, request, session
+from quart import g, make_response, request, session
 
 load_dotenv()
 
@@ -390,3 +391,27 @@ def slugify(text: str, max_length_chars: int = 80) -> str:
     text = re.sub(r"[^\w\s-]", "", text.lower())
     text = re.sub(r"[-\s]+", "-", text).strip("-")
     return text[:max_length_chars]
+
+
+# =============================================================================
+# A/B Testing
+# =============================================================================
+
+def ab_test(experiment: str, variants: tuple = ("control", "treatment")):
+    """Assign visitor to an A/B test variant via cookie, tag Umami pageviews."""
+    def decorator(f):
+        @wraps(f)
+        async def wrapper(*args, **kwargs):
+            cookie_key = f"ab_{experiment}"
+            assigned = request.cookies.get(cookie_key)
+            if assigned not in variants:
+                assigned = random.choice(variants)
+
+            g.ab_variant = assigned
+            g.ab_tag = f"{experiment}-{assigned}"
+
+            response = await make_response(await f(*args, **kwargs))
+            response.set_cookie(cookie_key, assigned, max_age=30 * 24 * 60 * 60)
+            return response
+        return wrapper
+    return decorator

padelnomics/src/padelnomics/templates/base.html

@@ -20,7 +20,7 @@
   <link rel="stylesheet" href="{{ url_for('static', filename='css/output.css') }}">
 
   <!-- Umami Analytics -->
-  <script defer src="https://umami.padelnomics.io/Z.js" data-website-id="4474414b-58d6-4c6e-89a1-df5ea1f49d70"></script>
+  <script defer src="https://umami.padelnomics.io/Z.js" data-website-id="4474414b-58d6-4c6e-89a1-df5ea1f49d70"{% if ab_tag %} data-tag="{{ ab_tag }}"{% endif %}></script>
 
   <!-- Paddle.js -->
   <script defer src="https://cdn.paddle.com/paddle/v2/paddle.js"></script>