Expose updated_at on v_usdc_purchases view

Two reader routes (v1_users_purchases, v1_users_sales) return updated_at
in their JSON payload. Alias sp.created_at to keep the API contract
intact — the legacy table sets both columns to CURRENT_TIMESTAMP at
insert and never updates rows, so created_at = updated_at in practice.

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

Author: rickyrombo

api/dbv1/models.go

@@ -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

ddl/views/v_usdc_purchases.sql

@@ -12913,6 +12913,7 @@ 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