Expose updated_at on v_usdc_purchases view

v1_users_purchases.go SELECTs purchases_with_content.updated_at through
a CTE wrapping v_usdc_purchases. The view never exposed updated_at,
which crashes the /v1/users/{id}/purchases page with:

  ERROR: column purchases_with_content.updated_at does not exist
  (SQLSTATE 42703)

Alias sp.created_at as updated_at to keep the legacy API contract
intact. The legacy usdc_purchases table set both columns to
CURRENT_TIMESTAMP at insert and never updated rows, so
created_at = updated_at in practice.

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

Author: rickyrombo

ddl/views/v_usdc_purchases.sql

@@ -13,6 +13,7 @@ SELECT
     sp.content_type::usdc_purchase_content_type AS content_type,
     sp.content_id,
     sp.created_at,
+    sp.created_at AS updated_at,
     GREATEST(
         sp.amount - COALESCE(
             CASE sp.content_type

sql/01_schema.sql

@@ -9631,6 +9631,7 @@ CREATE VIEW public.v_usdc_purchases AS
     (sp.content_type)::public.usdc_purchase_content_type AS content_type,
     sp.content_id,
     sp.created_at,
+    sp.created_at AS updated_at,
     GREATEST((sp.amount - COALESCE(
         CASE sp.content_type
             WHEN 'track'::text THEN ( SELECT (tph.total_price_cents * 10000)