fix(billing): fetch line items for checkout.session.completed webhooks

_extract_line_items() was returning [] for all checkout sessions, which
meant _handle_transaction_completed never processed credit packs, sticky
boosts, or business plan PDF purchases. Now fetches line items from the
Stripe API using the session ID, with a fallback to embedded line_items.

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

web/src/padelnomics/billing/stripe.py

@@ -346,16 +346,30 @@ def _get_period_end(obj: dict) -> str | None:
 def _extract_line_items(session_obj: dict) -> list[dict]:
     """Extract line items from a Checkout Session in Paddle-compatible format.
 
-    Stripe sessions don't embed line items directly — we'd need an extra API call.
-    For webhook handling, the key info (price_id) comes from subscription items.
-    Returns items in the format: [{"price": {"id": "price_xxx"}}]
+    Stripe doesn't embed line_items in checkout.session.completed webhooks,
+    so we fetch them via the API. Returns [{"price": {"id": "price_xxx"}}].
     """
-    # For checkout.session.completed, line_items aren't in the webhook payload.
-    # The webhook handler for subscription.activated fetches them separately.
-    # For one-time payments, we can reconstruct from the session's line_items
-    # via the Stripe API, but to keep webhook handling fast we skip this and
-    # handle it via the subscription events instead.
-    return []
+    session_id = session_obj.get("id", "")
+    if not session_id or not session_id.startswith("cs_"):
+        return []
+
+    try:
+        s = _stripe_client()
+        line_items = s.checkout.Session.list_line_items(session_id, limit=20)
+        return [
+            {"price": {"id": item["price"]["id"]}}
+            for item in line_items.get("data", [])
+            if item.get("price", {}).get("id")
+        ]
+    except Exception:
+        logger.warning("Failed to fetch line_items for session %s", session_id)
+        # Fallback: check if line_items were embedded in the payload (e.g. tests)
+        embedded = session_obj.get("line_items", {}).get("data", [])
+        return [
+            {"price": {"id": item["price"]["id"]}}
+            for item in embedded
+            if item.get("price", {}).get("id")
+        ]
 
 
 def _extract_sub_items(sub_obj: dict) -> list[dict]: